It appears your fame, and your overall battle quality, will determine your ability to have returning Tales characters join your team permanently. "Most of the time, these 'guest' characters will be available to you for their associated quest, but you can also try to get them to join you permanently. Whether they do or not depends on their overall opinion of you, which is determined by a variety of factors that are so far unclear. We imagine you may not need to do much besides playing skillfully and avoiding death when they're with you on a quest."
If you play like a second-class warrior, you'll end up with second-class partners: generic heroes that must be paid to follow you around. Like in real life, it appears you'll have to earn friendship through charisma ... or money. See a video of the game, after the cut.