Hey bababubu1505!
About Diego and the romance you can have with him that’s actually something you can start exploring in version 0.4 of the game (the one currently available on Patreon).
Romance in Trueclash tends to be gradual, though.
As for version 0.3 (the one currently public here on Itch.io, which I believe is the one you played), you can already start getting along better with Diego there and even walk with him to the dorms.
I’m pretty sure Viv or Alex made a small guide a while back about one of Diego’s CGs, I just don’t remember if it’s something publicly available or one of the Patreon-exclusive tips.
Hope you’ll enjoy what’s coming next (I think you will <.<)