thanks! I'll try my best to answer, it's been a while and I don't remember all the things I did...
After every battle, every character has a chance for at least one random stat to go up by a little tiny bit - that includes characters not in the party and not discovered yet. So hopefully, the characters you find later aren't TOO underleveled compared to your active party.
In addition to that, every action taken has a chance of increasing its related stat a little bit - punching can increase strength and max HP, using a gun can increase agility and luck... I forget which actions directly affect which stats, but it's not a 1-to-1 thing, made to avoid the Final Fantasy 2 problem where the only way to increase defense was to get your ass kicked.
So yeah, a non-gun-using character's gun-related stats will go up a little sometimes, but not nearly as much as a character who actually fires a gun every battle.