Yeah, there were some very cool characters in Chrono Cross that we coulda learned more about. I can't remember their names now (I played and beat it within a month of its release), but I do remember Karsh, Glenn, and some other army guy that were all great. The one rockstar guy DID have a lot of side plot/quests, so that was awesome. He was one of my favorites.
It's pretty funny when some of the side characters are more interesting than the main characters like Kid.
I can't think of any others right now even though I know they're out there. Games like Eternal Darkness excel in the side character area. There is one main character, but you only play as Alex for a small portion of the game. She's awesome, but some of the other characters were simply amazing. All of the side characters were playable and had unique attributes, dialogue, weaponry, stories, etc. I love that game.
-Lasereth