This seems like an interesting thread, and I'm kinda surprised no one has posted in it yet. Anways, here's some of my character flaws/quirks
Crispin: Crispin has the shortest fuse in the world; he gets angry so easily. He was also raised by an asshole pirate father who told him that crying is a sign of weakness so when he's upset he bottles it up and takes it out on other people (usually Kit). As far as quirks go, his feathers will ruffle when he gets really mad or annoyed. Also low key has abandonment issues which comes from his mother abandoning him as a child and his father trying to kill him at the start of the book.
Kit: Kit is the crown prince so he obviously grew up really sheltered and privileged. His biggest flaw is just that he's really painfully oblivious, especially when it comes to how other people live their lives. Like he's genuinely surprised to hear that Caleb makes like $3 per day and has to hunt for his food because he can't afford to buy it. He's also just really entitled sometimes. Like as soon as someone anything remotely negative about him he's all "You can't talk to me like that."
Caleb: Caleb daydreams a lot mostly to escape the pretty awful life that he's been living. He's a little bit naive but that's kind of understandable since he's only 14 and has never left his hometown. Has a tendency to sometimes get his hopes up a little too high which sometimes leaves him disappointed.
Gerard: Gerard has a really hard time trusting other people. He's super paranoid and will almost always jump to the worst possible situation. Hates change, and has like 0 backbone. Most of these traits come from his really tragic past, which he can't let go of. He works as a healer, but takes on a lot of side projects to kind of keep himself distracted so that he doesn't end up getting super worked up and emotional over things that happened years ago. Also has the world's most erratic sleep schedule.
Matthew: Matthew is a really interesting character to me but I can't really reveal a lot because of spoilers. Anyways, he's the king, but he's also really antisocial and reserved. He has a lot of insecurities, and has a tendency to shut people out (including Kit). Cares way too much about what other people think about him (which as King is probably not such a bad thing, but he kind of lets it get to his head).
Dallas: Dallas is really just kinda jaded and complains a lot about all kinds of things. He's also more on the introverted side (which isn't really a bad thing in itself) but he sometimes gets drowned out by stronger personalities like Ara and Andor and often gets roped into things that he doesn't want to do (the story literally starts with him at a party that he didn't want to attend). He'd basically rather just act as a bystander than step in and take action in a lot of situations.
Ara: Ara is really smart (which also isn't a bad thing) but he tends to act like that makes him better than everyone else. He's really snarky and condescending, is super bossy, and would rather die than admit he's wrong about something. Also drinks an unhealthy amount of coffee. Oh, he's also really judgemental too.
Andor: Andor is really just a disaster. Says and does things without thinking, hates being told what to do (So I'm sure you can imagine how well he gets along with Ara– He likes to say things just to get a rise out of him). Kind of lazy, procrastinates a lot, doesn't have his priorities sorted. He also has ADHD, so some of these traits stem from there. He also grew up in a strict household some of them also stem from him wanting to rebel a little bit.
(I'll go over some more of my characters later. It took me like an hour to write this and it's like 12am right now).