He joins for several random dungeons throughout the game, but yeah he doesn't fully join until the final dungeon.

Is Flynn playable Tales of Vesperia?

Flynn Scifo (フレン・シーフォ, Furen Shiifo^?) is a major character in Tales of Vesperia. ... In the Xbox 360 version of the game, Flynn is a guest character participating in a single fight, but he is promoted to fully playable in the PlayStation 3 port and Definitive Edition.

How do you get Patty in Tales of Vesperia?

Patty joins when you get the boat, but then leaves until the sands of Kogorh. Flynn joins before the final battles of each act. Naturally Flynn isn't considered permanent until the end of act 3.