Here's some Luci Headcanons for you! I could give you hundreds but I'll supply the dead Cerbie content with a handful that included the loveable three headed beast.
I hc Lucifer never wanted a pet because he already has so many responsibilities. He only lets Levi have one because he's shown responsibility & he knows how brother needs a companion. He doesn't hate cats, he hates that Satan goes overboard & then he has to step in & have more living beings to take care of. (There's a text in the first game I think when he tells everyone to leave a stray cat that got inside alone, he's not a cat hater he's just a hater of more responsibilities.) In nightbringer when mammons plan inevitably ends in Lucifer saving them from Cerberus & thus becoming the three headed beasts owner he's at a total loss. He's never had a pet be4 in his entire life, the amount of questions he likely asked Diavolo before taking the beast & then the amount of research he did after taking Cerberus just to ensure he knew how to properly care for him is insane.
I hc at first he didn't have Cerbie guarding the grimoire in the tombs at first. He didn't think he could actually trust the beast even though it had chose him as its owner. It wasn't until Cerbie proved himself worthy by protecting Lucifer from a random dumbass demon while on a walk that Lucifer began to trust Cerberus, as the trust grew, the things he allowed Cerbie to be in charge of protecting grew too.
I hc that no matter how late Lucifer stays up at night or even on the nights he stays up all night, he does a patrol of the house to ensure everything is all in order. He likely has specific time spans he goes & does this (switches them at random so his brothers can't just work around his schedule) when he's done checking on his brothers he always heads down to the tombs. He knows nothing will be there, Cerbie is a good boy who ensures that, he just wants to give the beast some attention he deserves without being disrupted or made fun of by his brothers. Some nights he may even take his work down to the tombs & spend the evenings with Cerberus.
I hc that every time he's too busy & has to make Mc or Beel take Cerberus for a walk, he feels really guilty, he may not be able to make it up in walk form for a few days til his schedule calms down but he always comes home with better treats than normal & spends extra time giving Cerberus attention.
I hc that he doesn't always like that Cerberus lives in the tombs. So on nice weather days Cerbie is allowed to play in the gardens, I mean he's home who's gonna mess with the grimoire right? When it's not the best weather he may on occasion use a spell to allow Cerbie to shrink in size so the three headed beast may fit wherever within the house. (Done mostly when Mc complains about Cerberus being lonely down there but sometimes he just does it because he feels bad for the pup.)
I hc personally that my mc would give Lucifer shit for how Cerberus is treated (with love that's my husband ong hes just a bad pet dad sometimes) I think hearing that he's leaving Cerberus uncared for & treating him as a tool at times would make Lucifer's pride take a hit. In his defense he's never owned a pet before & he's only ever known love in certain ways, protect, provide for, keep alive. Still after hearing Mc say that he strives to do better not just for his pride but because Cerbie is a living being & does deserve better.
I hc that on days when he's overwhelmed or just wants a five minute break he sneaks down to the tombs & uses Cerbie as a buffer. None of his brothers will find him when they won't be able to get past Cerberus. Lucifer doesn't mind that Mc can make it past Cerbie & find him, though he was embarrassed the time Mc found him napping while lying with the three headed beast. He had a serious migraine okay? He didn't mean to fall asleep it was just quiet & comfy.
I hc that Ceberus is trained to never actually hurt Lucifer's brothers or Mc. Sure he'll growl, he'll chase, he'll make them think he would because Lucifer trained him to keep them out of the tombs, but he also ensured Cerberus knew harm was completely off limits for those he cares about, trap them in a corner, chase them back upstairs, bark loud enough for Lucifer to come down, but never ever cause harm. If it's some stranger that's a completely different story.
Some Ido hcs I have as well, when Lucifer & mc move out they take Cerberus with them (love the game but where tf is my Cerbie mention. That's our child bro we ain't leaving him!) Cerbie doesn't live in tombs anymore he still guards the grimoire because Lucifer brought it with & it gives the three headed pup a purpose but he also guards the house itself more. Cerbie has a nice place to stay in my Mc made sure of it, however he's also turned into a more sizable beast much more often so he can frequent the house as much as he wants. Lucifer made the spell be where if Cerberus feels the need to protect like from a stranger that he'll be able to turn back to his normal size at any moment.
I hc that when Mc is too busy working at RAD & hasn't been home much, Lucifer hasn't seen his brothers or Diavolo much, he'll seek out Cerbie. They'll spend the day doing chores with Cerbie following Lucifer around. When they finish perhaps Lucifer will give Cerbie a treat & listen to a cursed record with a glass of demonous as he waits eagerly with the three headed beast for his spouse to come home. Mc has come home to find the two lying asleep on the couch together, it's a wonderful sight.
I hc that Lucifer has become much more lenient on Cerberus (perhaps Mc's influence) he allows the beast to do more things than normal. Before marrying Mc, Cerbie wasn't allowed on furniture & mc & the beast would get yelled at when Lucifer would find the human allowing the beast on the couch or their bed. After marriage Lucifer stopped being as forceful of the rules. Perhaps he felt pity, or maybe it was one of the days he was feeling lonely without his spouse, he let Cerbie up on the couch & the pup has his own spot on the couch now ever since. It was probably because he accidentally let Cerbie get hurt, or maybe Mc was the one that encouraged it- but Lucifer on days he's feeling in an especially good mood will let Cerberus sleep on the bed with him & his spouse. He low-key gets jealous when the beast makes him unable to cuddle his spouse throughout the night. Def trained Cerbie to sleep at the end of the bed or at least out of his way of his spouse. & Don't get me started on the fact that this man went from no table scraps no normal food besides meat, bros brothers nearly lost their marbles when they came over for dinner and watched Lucifer feed half his plate to Cerbie who was nosing at his knee. Between Lucifer & Mc indulging the three headed beast Cerbie has gained weight since the two wed. Lucifer has spare time and takes the pup on many walks to try & keep him healthy.
He likely never did it before Mc came along, but after Mc & especially after marriage Cerberus is given gifts. For the holidays & especially the beasts birthday, Lucifer spoils Cerbie only giving the best to what has become him & his spouse's fur baby. Bros got more bling & fancy food than Mammon or Beel could dream of. It was mc who did it first, Lucifer thought it was ridiculous, buying the three headed beast outfits & his own blankets, pillows, plushies, whatever. However it quickly grew on Lucifer, while out shopping he'd be like "hm Mc would like this one & this one seems fitting for Cerberus's taste, he's taken quite a liking to fluffy items with duck themes" mc turned him into such a pet dad.
Before Mc came along he never once called Cerberus anything other than his name or a beast when referring to him as what he is. He doesn't even remember when he started calling Cerberus things like Cerbie or Cerb, a pup or his fur baby it just seemed to happen on its own. He only uses Cerberus's name anymore when the beast is in serious trouble. & If you don't know Lucifer you may come to believe he owns a tiny puppy instead of a large three headed beast with how he talks about Cerbie.
Lucifer's pride would never let him admit it, but after marrying Mc he's found that in private he sometimes talks to Cerbie in a baby voice (stupid Mc & their bad influence!) he only does it when he knows the house is empty, the embarrassment of Mc coming home from work to that is something he can't live down as the human teases him relentlessly for it.
Okay I'll stop I'm sorry if I gave too many i just like to ramble especially about them lol. I hope you enjoy my hc's genuinely an endless amount I could give. I have plans to draw my hc's & content I've written for obey me out just been busy ugh not enough time in this short life! I love this man & our fur child who the game has forgotten about :" have a good one! Lemme hear any Cerbie hc's you have if you have any lol
OMG CERBIE HEADCANONS YES
I fucking LOVE Cerberus, and I HATEE that mc is canonically scared of him:(
I need MC to be sit in their room with a smaller cerbie on their bed and one of the brothers walking in all āhey mc do you know where- ACK-ā and cerbie doesnāt want anyone around mc while theyāre giving him attention<3