Does Devlin experience PTSD or shell shock whenever he hears gunshots, fireworks, or sudden loud noises? If yes how does he deal with it? Does Cian help him through it?
Whew this got long. Answer under cut.
Devlin has extremely severe PTSD. However, none of it is from war...but he sure has a whole lot of nostalgia from war. So much so that he has forged false identification documents so he could enlist and deploy to fight in multiple wars since becoming a demon. Dev did not join the military because of patriotism or because he needed the money or anything like that. The entire reason Dev enlisted in the first place (he was not drafted, he had already enlisted prior to the draft) is because he absolutely loves chaos, danger, and killing. He had been getting into gunfights since his preteens and had been hit with a bullet on six occasions before joining the Marines. Enlisting and deploying in active combat meant he got to slaughter people en mass without having to hide bodies and evade the law — quite the opposite, he got paid and praised for it.
Dev is not in any way a good person. He was born with more than a few screws loose. However, between the ages of 6 to 7, he went through an extremely traumatic experience which was ongoing and lasted almost a full year. It was after that he began to torture animals, harm other children, and it wasn't long before he came to Octavian asking for help hiding a body. What happened to him is not something he ever talks about and almost no one even knows about it, let alone any details. But it very clearly completely destroyed the person he had been and left nothing but a husk filled with darkness and rage.
The closest he's ever come to disclosing what happened to him was when he was extremely drunk and told Cian that "the trenches may as well have been a vacation compared to that damn room," after Cian had asked if the trenches were as bad as what he read. Cian asked what he meant and Dev just ignored the question and excused himself to the bathroom. After 10 minutes or so had passed and he hadn't come back, Cian knocked on the door and got no response so he busted it open and found Dev unconscious on the floor. Obviously he was a demon at this point so he couldn't die, but Cian thought he might be dead for a bit due to the state he had found Dev in. That incident in particular played a huge role in contributing to Cian's clinginess and worsened his anxiety of being separated from Dev even for a minute or two.
Octavian did tell Cian everything he was aware of after that, which is the Dev had wandered off when he was 6 and presumably was kidnapped and held prisoner for nearly a year. The scars Dev has were enough for Cian to be able to tell he had been very badly hurt at some point in his life and Octavian did confirm to him which scars were from injuries Dev had at the time Octavian found him. Octavian stressed to Cian to never bring up the topic to Dev, and Cian never has brought it up.
Cian always has done his best to comfort Dev when he can tell that Dev is in a bad mental place, he gets very quiet and will have a very blank expression while staring off at nothing in particular. It can be a bit tricky since Dev often won't respond in any manner if spoken to and touching him is a 50/50 gamble of snapping him back to reality or sending him into a full blown mental breakdown. Cian is very careful to avoid the things he knows are a trigger for Dev, but sometimes the break downs happen seemingly out of nowhere and with no warning. He does his best to minimize the damage without making things worse when Dev has his breakdowns. The tricky thing is that physically restraining Dev in any manner will make things 100x worse, but if no one does, Dev will wind up becoming extremely destructive and hurt himself and/or someone else very badly.
So, yeah, Devlin does have severe PTSD, just not from war. By the time he was deployed, he had already reached the point of being so broken that there wasn't anything left to break. See, he got the news Cian had hung himself right before entering active combat — he thought Cian was dead. Dev was a man with nothing left to lose.