How to Play: Kirby
A short and incredibly niche one today, because... Pathfinder and 5e just don't seem to really support the ideas I'm using to build Kirby (or they do, but at a ridiculously late level, or in a really poor way, I'll talk about those)
Ok, so, a pink marshmallow who floats around, sucks up enemies, and steals their shticks.
You might be surprised at how relatively easy this is to do, in 3.5, at least.
Stealing Powers
We'll start with stealing enemy shticks, because that's very straightforward- you play a spellthief (CAdv13. That's it, that's literally the point of the class. It's not the most powerful, but I think you can pick some things up as you go to make it a bit better, while also making your character more like Kirby.
Floating Around
The next quintessential Kirby ability is floating around. There's a relatively direct way to do this, you take a level of Elocator(XPH142), which gives you the ability Scorn Earth, allowing you to float 1' above the ground at will. It will require you to take the feats Dodge, Mobility and Spring Attack, as well as having some way of manifesting first level powers, and the two best ways, so far as I'm aware, are a two level dip in Psychic Warrior (two levels because you get a bonus fighter feat at each of those levels, allowing you to handle two of the Elocator requirements), or taking Hidden Talent (XPH67), which just straight up gives you 2 power points and a single known power (I would suggest Catfall, for flavor reasons).
But it Could Be Better
So, on my first go through, I kinda left this section at that, but with more detail and discussion on those two options, but then I found a better option that actually represents Kirby's floating around better.
Starspawn (LoM) gives you a flight speed equal to half your land speed, that you can use for (Con Mod+1) consecutive rounds, minimum 1, with 1 round of not flying between uses. It also gives your cold resistance 5 and immunity to altitude sickness and altitude fatigue, which... neat. It requires Aberrant Blood and one other Aberrant feat, but this is not a downside (see Get in My Belly), especially when the alternative, Elocator, requires three feats and a detour from your spell stealing ability.
A Psionic Spellthief?
I think there's a potential a "Psionic Spellthief," where you replace the spells that Spellthieves gain with something like a delayed Psychic Warrior progression manifesting, but I feel like it would then be logical for the steal spell ability to become "steal power," giving you the ability to steal psionic powers rather than spells, which is probably not going to be anywhere near as useful. However, that said, if your GM sees psionics as just a flavor of magic, then maybe it would be a simple "cut spell progression, paste psionic progression," that doesn't alter your main class ability. But if your GM handles psionics that way, then you'd be better off asking if you could change elocator to be magic (requiring the ability to cast 1st level spells, and improving spellcasting), rather than psionic. Which would be much better than making Spellthief psionic, and might even give you reason to take more levels of Elocator. And, again, Elocator is the less accurate, less good way to do Kirby's floating, so.
Get In My Belly!
Ok, so now you float around and stab people and take their shticks. What about the "swallowing people whole" part? WELL... outside of his own games, Kirby's swallow ability is more like a grapple than actually swallowing people, which I think is a good way of doing it in D&D. But we're also going to see if we can work out something with the same end result of literally consuming your enemies.
Making this build the best it can be is going to be rather feat intensive, so, if you can take Flaws (UA91), that would help a lot, but it's a variant rule, so it's really up to your GM. If you are allowed to take flaws, I suggest Poor Reflexes (-3 Reflex saves) and Vulnerable (-1 AC), because Kirby feels like a guy who is really easy to smack, and is more durable than evasive. Make sure you have good hp if you do this. If you'd rather be more evasive than durable, then I suppose you could take Frail (-1 hp/level) and Meagre Fortitude (-3 Fort) instead, but... that's not how I see Kirby, and I think being less durable is going to hurt more than being less evasive. But it's certainly an option.
Feats
We're going to start by talking about what was originally a "this is nice" feat, Aberrant Blood, because it enables the better "float around" method. Aberrant Blood is the foundation of the Aberrant feat group, feats which represent your character being descended from aberrations. When you take it, you select one of a handful of bonuses to get from it, and the one of particular note is Flexible Limbs, as it grants you a +2 racial bonus to Grapple. None of the other bonuses are directly useful to "Being Kirby," though I could see Slimy Skin being a good hedge against the possible chance of grappling someone who's better at it than you, as it gives a bonus on Escape Artist. But, honestly, improving your grapple is going to be a better hedge against that than trying to build up your Escape Artist in addiiton.
Working on the idea of increasing your bonus to grapple as much as possible, a frontrunner in that goal would be Improved Grapple, which requires Improved Unarmed Strike, which isn't great, but, hey, it could be worse. If you decide to go the Elocator route, it's definitely going to hurt. But I suppose, whether you go Elocator or Starspawn, you're spending three feats to get off the ground, so, kind of a wash, I guess.
If you do take Improved Grapple, Scorpion's Grasp (Sandstorm), which requires it, would be a good follow up, as it allows you to immediately initiate a grapple when you hit an opponent with an unarmed, light or one-handed melee attack. You will have to drop your weapon if it's one-handed, but if you're using light weapons, you're all good, albeit either way, you're forgoing the option to make further rolls to deal damage on the round you start the grapple. So, when you have iterative attacks, you're going to want to leave the grapple initiation to your last attack in a round. This does suggest dipping monk, since it will give you Improved Unarmed Strike and mean you can grapple first and damage after on the same round, which does feel more fitting. That dip also allows you to take Improved Grapple as a bonus feat at first level, so, it's definitely compelling, especially as a single level dip.
Grappling handbooks will suggest Open Lesser Chakra (Arms) (MoI40), but a feat that requires you to be 12th level before you take it, especially for a +2, is just such a late, bullshit thing that I, personally, definitely cannot be arsed to care about this weird alternative magic supplement just for that. That said, Shape Soulmeld (Girallon Arms) (MoI40) requires just that you have a Con of 13, and gives you a +2 bonus on grapple (and climb) checks, which increases by 2 for every point of essentia you put into it (which is capped at, basically, 1+1/five levels after first). It doesn't give you any essentia, so you may want to look at ways of getting it, like Bonus Essentia, or playing one of the races from the book, but honestly, just having the soulmeld gives you a +2 competence to grapple, which is very worth it if you can spare the feat. There's also the soulmeld Kraken Mantle, which will give you an untyped +(invested essentia) to grapple if it's bound to your arms chakra. So... if you go ahead and take Open Lesser Chakra, then I suppose Shape Soulmeld (Kraken Mantle) is... ostensibly worthwhile? But, again, I feel like, unless you're playing an incarnum class, it's just way too late, for not enough bonus (though, I will say, if you have the essentia, this would give a +4 untyped bonus to Grapple that stacks with everything).
One last feat worthy of mention here is Jotunbrud, which allows you to make grapple and other strength-based opposed rolls as if you were large, regardless of your actual size. My inclination is to make this character a goblin, because it feels like it just fits, but Jotunbrud, as written, requires you to be a human. I think either choice is defensible, either a goblin for the feel, or a human, with the lowest height you can get your GM to agree to, so that you can take Jotunbrud, is good. If you just care about mechanics, playing a 4'5" human with giant's blood is honestly superior.
One side note is that... as much as I hate it, and as much as it would kinda suck because it doesn't directly feed into your grapple build and requires another feat to take, Defomity (Obese) (BoVD48) would be a very fitting feat for this character. I hate the feat because it's just... bad taste. But, it increases your Con, so you get more hp (though the various toughness feats would do this, and some to much better extents), and it makes you look more like Kirby. However, it requires Willing Deformity (BoVD49 or HoH125), which also doesn't improve anything you're specifically doing here. So, it's another matter of doing a less good thing for fluff. Honestly, for the fluff aspect, just ask your GM if they're cool with you playing a 4' tall, roughly spherical character, who isn't getting any specific mechanical benefit for either of those things. If you do want to go this route, then I suppose consider also taking Deformity (Hands) (BoVD) or (Teeth) (HoH), as it gives you a natural attack (claws or bite, respectively), which could be useful with Scorpion's Grasp. And a bite would be fitting. However, Vile feats, such as the deformity feats mentioned here, do require your character to be evil, so just keep that in mind.
Get Over Here!
Now, the last part of this puzzle is Kirby's ability to inhale people, which, mechanically, is actually the ability to grapple people you're not standing right next to. The main way I can think of to do this is another Aberrant feat, Inhuman Reach, which improves your natural reach by 5' (to 10' for most characters). Now, granted, it also inflicts a -1 penalty to your melee attacks, but... well, one, that's very minor, but also, two, it feels like Kirby is not particularly melee oriented, so this -1 feels on brand. Inhuman Reach also helps you qualify for Starspawn, as it only requires Aberrant Blood itself. It is worth noting that, the way D&D's rules work, natural weapons threaten any space you can reach, even bite. So, while, technically, the feat is about being generally stretchy, if you have a bite from some source or another, you actually can just reach your mouth over 10' away and bite someone.
After you take Inhuman Reach, you can then also take Extended Reach (SavS34), which will extend your reach a further 5', meaning you can grab people 15' away.
Also, with the way grapple works, you move into their square, meaning you get a free movement equal to the distance between you and your target. I feel like there should be a way to move your opponent into your space, but I don't know of one off the top of my head.
I Accidentally The Whole Thing
Now, if you want to actually, fluff-wise, swallow someone and take them out of the fight, then, mechanically, you're just dropping them. So, really, what you want to do, is chase enough damage on your grappling attack that you can reasonably expect to drop an opponent, at least one of comparable level to you. Given that Steal Spell works off of sneak attack, that's reasonably simple enough. Sneak Attack is already a very good way to increase your damage. Then, also grab two-weapon fighting, so you can make more sneak attacks in a round, and, if you have multiple natural weapon from whatever sources, consider taking Multiattack, and figuring out how multiple natural attacks, manufactured weapons, and iterative attacks work out (I don't actually recommend it. Life is too short to work out that clusterfuck, that's why I'm not telling you how they do, except to say "it's a pain in the ass").
If you dipped Monk, then you already get Two-Weapon Fighting, just specific to your unarmed strikes and special monk weapons. Of note is that... Flurry of Blows, while it works exactly like Two-Weapon Fighting, it doesn't say it is TWF, which means you can use them together (though the penalties will stack). This means that, if you are, say, a 4th level monk with TWF, then you get a total of three attacks, each with a -4 penalty (or, basically, BAB -1). If you take additional levels of monk, then they will give more attacks with, and reduce the penalty of, Flurry of Blows, and if you take Two Weapon Fighting, then, strictly, as written, you need to take the follow up feats to continue getting additional attacks as your BAB increases and gives you iteratives (in my games, I basically replace TWF with Perfect Two Weapon Fighting, saying that you always get an equal number of attacks with your off hand to you primary hand).
That's going to be your main way of increasing your damage. But there are certainly other things that will help, such as strength enhancing items, maybe raging, and so on. It might be worth noting that Ascetic Rogue (CAdv106) exists, and if you have an unarmed strike, then you do qualify, but, as written, you'd have to ask your GM if you can stack Monk and Spellthief when it comes to the increased sneak attack and unarmed damage. If you're interested in more than a one level dip in Monk, then it might be worth checking, but honestly, I don't, personally, think there's a lot of compelling reason to do more than a one level dip in Monk.
Extras
I haven't totaled the feats, so I don't know how much playing around you're really going to have (especially as that's going to depend on your precise build), but it's worth looking at some things that, while not directly advancing your "float around, grab people who aren't standing next to you, and steal their stuff" deal, are quite fitting.
In Aberrant feats, there is Durable Form, which gives you 2 hp per Aberrant feat you have, and Bestial Hide, which gives you a natural armor bonus equal to half your Aberrant feats.
If you decide to include Incarnum stuff, then Bonus Essentia would be very handy, giving you 2 extra essentia points--because if you're not getting Shape Soulmeld, then you probably don't care about having Essentia (without the ability to shape soulmelds, it only gives 1). Of course, I say that, but there is Azure Talent, which allows you to invest essentia to get bonus power points if you're pursuing the psionic build, and Midnight Augmentation, which allows you to just plug essentia into psionic power augmentation (tho I feel obligated to warn you that the amount of fiddling involved in doing so- obtain psionic focus, invest essentia, expend focus- your party will want to clobber you over your actual head so they don't have to sit through your turn). Indigo Strike allows you to invest essentia to improve your sneak attack, and Incarnum Spellshaping allows you to learn Incarnum spells, but personally, I feel like the Incarnum spells a Spellthief can actually learn are pretty thin gruel. There are spells that could be worthwhile, such as Soul Boon, and it does give you +1 essentia, so, I suppose if you have a feat to spend and nothing else to do with it, it's not a bad choice.
Illithid Grapple (CPsi61) would give you a tentacle attack, with which you have Improved Grab, but it requires Illithid Heritage, which itself requires you to have a power point reserve. Kirby doesn't have anything like a tentacle, but if you're going with a slightly looser "inspired by" build, the feat does say the tentacles come out of your mouth, so it would sort of fit. You can take this feat up to four times, gaining another tentacle each time (and, technically, once you have four, your lower face transforms to look like an illithid's, but that's fluff), and... it has a Special clause that talks about taking a full-round action to forgo any other attacks you have to make an attack and grapple attempt with each tentacle you have, but it's not clear that you can't... already make a full attack using each weapon you have and your tentacles, while initiating a grapple with each tentacle that hits, which is what the main text of the feat describes. Overall, I think taking it once could be good, as, while the tentacle does not increase your reach itself, it does operate with your full reach, but more than once makes things a bit confusing (which is a bit of a hallmark of late 3.5). The only real reason to take it more than once is that Illithid Extraction requires you to have it four times, and it allows you to attempt to extract an enemy's brain if you latch on with all four tentacles. It won't outright kill an opponent, but it will deal 2d4 Int damage to them, and if they hit Int 0 from this, they do instantly die. And this could work for a sort of "dark fantasy take on Kirby." It is worth noting, however, that sneak attack damage is the same type as the weapon that deals it, and characters in a grapple are denied their dex bonus to AC, meaning they're valid sneak attack targets. So... you're going to have to defend it, but, technically, Illithid Extraction on a character with Sneak Attack deals 2d4+Sneak Attack dice Intelligence damage. Meaning that your targets are probably gonna die. But then, you're probably at least 15th level at this point, and this relies on successfully making four attacks and, like, six grapple checks, so... it's probably fine, and you probably have better ways to deal int damage to your opponents earlier on if that's a thing you want to do.
On that note, actually, there's a feat in Book of Exalted Deeds, Touch of Golden Ice, which allows you to inflict the ravage Golden Ice with your touch, including natural weapons. Ravages are poisons that only affect evil creatures, and Golden Ice deals 1d6 dex/2d6 dex. It has a save DC of 14, but with the number of attacks you can make, and specifically unarmed/natural weapons, that's a lot of saves a target would have to make in a round, and if you reduce their Dex to 0, they're helpless and can be coup de graced, which would support the "I literally eat them" fluff. There's also the ravage Jade Water, which has the same save DC, and is cheaper than Golden Ice, but deals 1d4 wis/1d4 wis and 1d4 int, so you could probably get approval for "Touch of Jade Water" if you wanted to support an Illithid Extraction build. But, either way, your character has to be good, this tactic will only work against evil creatures, and you have to ignore the cognitive dissonance of "I am a being devoted to Goodness, and I use a magical power that literally inflicts horrible, torturous pain on my enemies, because they're Team Red." It could work, and I might look at it for a game where my character starts at epic level where I could be almost positive that my enemies would all, or nearly all, be evil.
So, What Does Kirby Look Like, Build-Wise?
Personally, I would go the more straightforward route, focusing on Spellthief, with a dip of Monk. I'd probably also start monk. So, something like Human Monk1/SpellthiefX, with the following feats in roughly this order-
Improved Unarmed Strike (monk bonus), Improved Grapple (monk bonus), Aberrant Blood- Flexible Limbs (human bonus), Jotunbrud (1st), Inhuman Reach (3rd), Starspawn (6th), Scorpion's Grasp (9th), Shape Soulmeld- Girallon's Blessing (12th), Open Lesser Chakra (15th*), Bonus Essentia (18th)
And if I'm allowed Flaws, then shift everything up to Open Lesser Chakra one space closer to 1st level and add in Extended Reach as another bonus feat at first level. If I could wring some more feats out, maybe a dip of Fighter, I would take Two Weapon Fighting early on.
*- Alright, fine, maybe take Open Lesser Chakra- Arms, since the place where I can fit Shape Soulmeld into the build is 12th level
What About Pathfinder/5e?
5e does have a spellthief thing, but it's a 15th level ability for a Rogue spec, which is far too late to base your character fluff around, unless you're starting at or above that level. I don't know if 5e supports the other parts of this idea, because once I found that, I didn't bother looking at a 5e build.
Pathfinder has something like the Spellthief's Steal Spell, but it's an Arcanist greater exploit, something you get at 11th level, so, much the same (unless your GM allows 3.5 material, in which case, it's the same build, but with maybe some tinkering you can do due to archetypes). There's also some... homebrew? I think? archetype for Rogue that makes it a spellthief, which would make the concept work, but, again, I think if your GM is allowing homebrew, then you can probably get approval for 3.5 material, and thus do basically the same build as 3.5.
Source Abbreviations
CAdv- Complete Adventurer
XPH- Expanded Psionics Handbook
LoM- Lords of Madness
MoI- Magic of Incarnum
BoVD- Book of Vile Darkness
HoH- Heroes of Horror
SavS- Savage Species
CPsi- Complete Psionic
