The thing about trauma is that everyone responds to it differently, so exhibiting trauma is going to largely have to do with who your character is, and what their personality was like before the trauma, or how it formed because of the trauma.
For example: both my characters Alicia and Kattar didn’t grow up with their fathers, but a number of factors affected the way they dealt with and displayed that trauma.
Kattar was spoiled and pampered by his mom, while Alicia was neglected. This led him to have “smiling depression,” (if your character is a cheerful person despite their trauma, perhaps research this.) He felt it was his job to PRETEND to be happy because his mother had done so much for him, and he didn’t want to make her unhappy. On the other hand, Alicia knew she wasn’t really valued or cared for, so she displayed symptoms we typically associate with depression.
That being said, again, trauma is complex, and everyone has lots of different reactions to trauma that manifest themselves in many areas of their lives, beside obvious PTSD symptoms like tremors, night terrors, and night sweats, anxiety, and depression (though as a starter, you can research these, and there is plenty of information on what those things look like.)
So think about how your character specifically reacts to their trauma and how that is relevant to the story, because you’re probably only going to see it if it’s applicable or relevant to the plot anyway.
Example: Kattar grew up without his dad, and his mother always complained about his dad, and men in general being useless. His reactions to this trauma were both for him to want to be a very independent person so that he was never “draining” or “leeching off of” anyone. And also to be hesitant to express his feelings because he was worried his best friend, who really admired his mom, would have absorbed his mother’s mindset and believed that men were a waste of time.
This isn’t how everyone reacts to trauma. I’ve dealt with very similar trauma to my character in pressure to perform, and also negative statements about my gender, but while it has led me, like Kattar, to try defy the idea that I’m useless, it hasn’t really led me to be hesitant and instead to be very “in your face” and matter of fact about who I am, whether people like it or not.
Now hopefully, you've not experienced your character's traumas, so you most likely won’t have a point of reference there. But even with a trauma that isn’t likely to happen in the real world, real people probably HAVE experienced similar things, so do a bit of research on things as similar to your characters as possible. If the trauma revolves around things that aren't likely to happen, then try to look for similar situations, for instance, perhaps his parents were killed by the villains, look up how abandonment issues characterize themselves, as well as PTSD for soldiers, and things like that. He's going to likely deal with a lot of similar issues to children who weren't raised by their parents, as well as the trauma of living in a dangerous situation where you watch people you care about be cut down. Chances are people will have had at least similar experiences to what your character has experienced.
I know this was lengthy, but hopefully it helps.
