Well, there's also how part of the war "stole" his brother away from him, but that isn't a plot point that is ever brought up back again, even though it's pretty traumatic.
I'm not gonna lie, I really wish there was more game specifically just to build some more on Reks and Vaan's relationship. On the one hand, they're brothers, it's not like it needs crazy expanding upon as to why that's horrible for Vaan. But on the other hand, it was something I was super interested in that ended up just being kind of buried in the background.
I don't know. I wouldn't want it shoehorned in if it wouldn't work, but as little as Reks is in the game I was always so intrigued with the more personal story that could have been between him and Vaan rather than the overarching political story. But alas, it is a political story at its core, so maybe I just don't know what I'm talking about haha.