To be honest Billy didn’t add much to the series so I don’t think he’ll be in the animate spin off other than be mentioned
babes, sweets, darling..... I need this to be satire or only abt s2 or smt. Cus if its not ur just... wrong? Billy was integral to the s3 plot no questions asked and to Max's storyline in s4. He didnt add lot in s5 bc the writers were shit at their jobs and just wrote Max's grief and trauma out of the plot. And while I dont think its his only purpose or true to who he really is, he added a lot to the kids' plot by being an in-person human antagonist they couldn't fight head on. Oh and he was also an antagonist who had personal connections within the party, which demagorgans and demodogs and even henry never really did, causing even more story beats to follow. Not to mention the complexities of his and Max's relationship that had so much potential.
I dont think Billy's gonna show in the spine off, not because he wasn’t important to the story (bc he so fucking was) but because I dont have faith in the writer's to see his potential(like they didnt the first time). I don't think they'd do him justice even if they did write him in. If they wanted to do Any interesting character work for Max they'd Need Billy but again, I don't think they'll write him back in with any amount of care or decency