During Disc 1 of the game, Quina is an optional character. S/he joins the party in the first disc only if the player travels to Qu's Marsh (Quina's and Quale's home) near the Lindblum Dragon's Gate. If you do not recruit Quina then, s/he can be recruited in the same place during Disc 2, at which point having Quina join your party is a mandatory requirement of finding a way to get to the Outer Continent, and making progress through the game.