All of them.
She might be in Kingdom Hearts Birth by Sleep.Not Kingdom Hearts.
Organization XIII's members definitely existed at this point, but the name 'Organization XIII' probably wasn't created until the time of Kingdom Hearts Chain of Memories when Roxas joined, making thirteen members altogether. We know, because of the events in Kingdom Hearts Birth by Sleep, that the members of the organization were mobilized, but "Organization XIII" in specific might not have been established.
No. He is not a playable character in Kingdom Hearts 2.
Probably depends on what the next Kingdom Hearts game is
The only playable character is Sora.
No because you haven't fought any members of Organization XIII in Kingdom Hearts 1, you haven't even seen a Nobody yet. In Kingdm Hearts 1 all of your enemies are either human or Heartless.
In the original Kingdom Hearts game, you can only play as Sora.
The only playable character in Kingdom Hearts Re:coded is Sora,herculis and cloud in the collisium.
The human forms of Organization XIII's members appear in Kingdom Hearts: Birth by Sleep for the PSP.
That's because in Kingdom Hearts 1 when Sora turns the keyblade on himself he turns into a heartless and that created Roxas as Sora's Nobody. Roxas became a part of the Organization of Nobodies Organization VIII and that's what brought the organization into Kingdom Hearts at all. But it was at the End of Kingdom Hearts so they put the organization in Kingdom Hearts Chain of Memories and Kingdom Hearts 2.
In Kingdom Hearts 2, Auron makes an appearance in Olympus Colosseum as an NPC who serves as a party member. Like all other party members, Auron is just an NPC (Non-playable character), and you can not play as him. The only playable characters in the game are: Sora, Roxas, and Riku.
No, He is not only Terra, Aqua and Ventus are playable