I know you are showing flashbacks to show character development, but I personally would like more current day development. Your character seems hell bent of revenge, because of bullying. Which I feel is rather one dimensional. To expand upon this, I would love to see Dan's every day struggles with life. Such as home, or work, or other social events. Rather that seething rage, what sets him off. What really bothers him specifically and why, did something happen to his mother to set him off. Often times people with anger issues have specific triggers, for example my dad would go off when something would break (typically the lawnmower or something to that nature), or even just working/chore issue in general, repeating himself too many times, etc... So, what sets Dan off, other that just a general hate. Make a specific list, such as "insults to one's family name', appearance, etc..." I suggest this so that it doesn't appear as vague, giving for character more depth.
I also know someone who is a general misanthrope, and has a extreme dislike for people. So, that individual generally extent little to no trust to others, of will always assume the words in people. Tending to avoid them or sass back like a monster. XD One of the reasons why they became that way was, yes, partially due to bullying, but they were also taught that all people are out to get you. And, the general menagerie of how other people interacted with each other. Sometime this thoughts and behaviors develop through observation of others (who may of may not even affect your life directly) in addition to your own issues.
As someone who had a similar bullying experience (except for the physical harm) You tend to float between anger, depression, and nothingness. I would love to see these explored more in your character's development. And as weird as it might seem, the nothingness is a very dynamic feeling. The apathy, the indifference, and self isolation feels like a black void.
I understand this is an action comic, but if you want to bring more emphasis on that action itself, use the down time to build up. Such as showing the character struggle with self control, or inner thoughts and dialogues. Body language. etc...
Action is cool and all, but I am more interested in the psychology of Dan. Think of the action as sprinkles on top. Because at the end of the day, a good fight is fun to watch, but it doesn't always move the plot forward. Like unnecessary romances. Character development will engage the reader, and then you can add the delicious sprinkles of action after.
Side note: I am not sure how old he is and I find that confusing because I though it said that he was maybe 10 at one point, and there there was a 15 year flash back, so that means he is 20 ish. XD
I hope this helps.