some final fantasy games have enigmatic characters who join the party for their own poorly-explained or unexplained reasons. Like Gogo or Vincent.
Final Fantasy IX, on the other hand, has Quina. Quina wants to eat new things. Quina does not want anything other than to eat new things. And frogs. Quina's motivations are not at all hidden or absent. They're just _weird_. Much like Quina themself.
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I guess ffix's enigmetic character is Amarant, but no one ever remembers Amarant. He's not really a particularly well-written or fleshed out character.
ffix spoilers
Eternally frustrated that Beatrix, a vastly more interesting character, doesn't become a permanent party member and is only in your party for like two fights
I guess they thought that two knights would be too much, and didn't want to drop Steiner, which, okay, yeah, Steiner is one of the better characters, but surely they could have made Beatrix more of a spellblade or paladin archetype (she even has paladin-like healing magic)
It's not like they don't already have two combo white-mage/summoners with dagger and eiko, anyway