I stupidly failed to get the Siren GF when fighting Elvoret, is it possible to get her later in the game?

Yeah it is, in Ultimecia's castle, im not sure which boss has it, but you CAN get ALL the GF's you failed to obtain during the game.Nice avatar btw.

Thanks =)

No problem.

Yeah! But by then it's kinda late to start Leveling them up and all.

fight tripoint in ultimecias castle to get Siren

It would probably be better to start the game again if you REALLY want siren throughout the game, since the castle is disk 4 and almost everywhere is closed on disk 4

That is true, but if you buy and get enough scrolls for the GF to learn everything then you should would be fine. You can then teach Siren most abilities that she has to learn.

Just start a new game. It isn't that hard. Really. Maybe you can get a better SeeD ranking too.