I remember during my first playthrough, I believed that almost every character with a portrait was going to join the team. It usually turned out to be true, especially if they also had battle models. These were the characters I was most disappointed about:
Sage of Marbule: Sure, he was not exactly excited about your quest, but characters joined the group with less motivation. While he certainly lacked his own Techs, he made up for it by fighting you with a mop. He seemed to be designed to join the team.
Fire Dragon: Really early in the game, the dragons seemed to be the good guys. It seemed very plausible that he would come into the team, and his weapon was really cool.
Dario: Who didn't think that after that battle he was going to join you and complete your little Deva collection? Too out of shape? Somehow I doubt that. His stance was an obvious reference to Frog, too.
So, who would you have liked to join the team the most?