Okay so we all know about my headcannon for Leviathan from Obey Me! having autism.
However...I have another headcannon for a different brother.
Beelzebub suffers from PTSD.
Oh yeah. ANGST...but also it makes a lot of sense.
When we first meet Beel, he's fairly grumpy/irritable. If I'm remembering correctly (which idk), I believe it's mentioned in game that Beel has a hard time sleeping, whether it's part of the story or text convos? I could be wrong but I feel like I remember this. He has flashbacks to the war, specifically losing Lilith, and we know he blames himself entirely for not being able to save both Belphie and Lilith.
All of these things can, in fact, be linked to Post Traumatic Stress Disorder. I wouldn't be surprised if all the brothers (excluding Satan who didn't exist at the time) had at least some degree of PTSD from the war, as often soldiers do have PTSD, but...I feel like Beel probably got the brunt of the disorder out of any of them.
So yah there's that headcannon. Leviathan is Autistic and Beelzebub has PTSD. At least that's how I personally imagine it.