Proving this isn't the worst idea ever.

Achilles moved around a lot as a kid, up until he moved into Hometown when he was around 12 after his mother's disappearance. He's autistic, trans, and gay, and he struggles a lot with connecting to people on a deep level. He was friends with the Holidays and Dreemurrs until Dess's disappearance when he was 16. Due to it reflecting his own trauma, he became incredibly distant with the two families - still staying in contact with Asriel, but generally keeping to himself.

He's very morally driven, wanting to do the "right" thing. Ever since Dess disappeared, his goal has been to find her and bring her back. There's a certain level of responsibility he puts on himself for this, feeling as though he should have done something to prevent her disappearance, to save her - but he's made little progress in the years since her disappearance.

If he were in the game, you'd be able to find him on the way to the Shelter, leaning against a tree. He'd be closer and closer to it, up until midway through chapter 4, where he's at the shelter, and you can ask him why he's always down here. he'd explain that a shelter is meant to keep people safe, and you don't just close off a location meant to save people. Something isn't right about that.

When asking about his scars (something you could do every chapter, every time you speak to him) he'd tell you a different story. In chapter 1, he'd say he's surprised Asriel didn't tell you, before pausing, and then having a little smile as he tells you an absolutely ridiculous way he got them. Susie likely says he's full of crap at some point lol

In the Dark World, his gimmick revolves around being unable to defeat enemies when he attacks them! He'll leave them at 1hp, and then he will purposefully miss attacks. He has a fire breath magic attack, a regular attack (which he does with a warhammer), and a healing action :> A-action would likely be some awkward social interaction and it would be funny every time.

Achilles is a person trying to adjust to "normal" life when he doesn't really understand normality, even after years of living in Hometown.

Zain, on the other hand, is a wanderer who intentionally leaves his backstory kind of nebulous. He just sort of showed up in Hometown one day with all his whimsy and smiles in tow.

Zain is cis, but he's unlabeled - whoever he's attracted to, he's attracted to. He's also poly! Like Achilles, he struggles to make connections - though unlike Achilles, this is 100% of his own accord, and he does it on purpose. Everyone has a nickname for him, and he doesn't call anyone by their name - unless they're Achilles, of course.

Zain has a strange relationship with religion, so much so that at the start of chapter 4, he likely can be found just leaning against the outside of the Church after worship. When spoken to, he'd say "oh I'm a devil, didn't you know? Going inside burns." Susie would probably immediately be fed up with him as he grins and messes with the both of them.

He's a shapeshifter, meaning he can appear pretty much however he likes - though he tends to keep the form seen above. If you ask Achilles about Zain at some point, he'd probably go into a time where he and Zain were hanging out, and he left to go get some food, only to be startled when he came back and Asriel was sitting there. Zain then started laughing, having turned into Asriel, and Achilles just. Holds his chest after that panic attack lmao.

Zain is also something of a flirt - though that wouldn't be something the main characters experience. The idea there's a dialogue option called 'flirt' and then instead of Kris ACTUALLY flirting, it's asking if Zain really does flirt with everyone, is really funny. He'd respond that he only flirts with people over 18, and while they're almost there, he's not quite comfortable flirting with them.

In the dark world, Zain would be very cowboy-based. His weapon would be a gun, and his gimmick would be he can mimic the abilities of his teammates! So hypothetically he could use Heal Prayer OR Rude Buster if you REALLY needed him to. Z-Action would likely be some sort of shapeshifting into the enemy, or something the enemy likes. He'd also have plant-based attacks alongside his cowboy themeing!

Sorry this got so long, I uh. These two are two of my most developed OCs, and so I have a lot of in-depth thoughts about them </3