I recruited Quina awhile back, and I already have Steiner, Garnet, Vivi and Zidane (obviously), but I realize there are two other characters in the game that able to be recruited. Quina, had I not gone into his/her swamp, I think I would have missed altogether.

My question is will I miss any other characters if I don't do a side mission somewhere or do they show up in the main story automatically? I absolutely hate it when I miss someone in an RPG so it's imperative I get them all (if that's possible).