Golden Sun: The Broken Seal & Golden Sun: The Lost Age: The party below:
The Big Bad - Saturos, the leader of the Apollo Sanctum raiding party and the Blue Oni to Menardi's Red. Karst is the Big Bad and also The Dark Chick in the sequel, wanting to avenge her sister's death.
Sixth Ranger - Felix, a crucial member whose loyalty is somewhat in question.
Worth noting that although this setup is active from the start, neither Agatio or Karst appear in The Broken Seal. With Felix's loyalty in question since he'd finished his main purpose (opening Venus Lighthouse), and with Saturos and Menardi dead, The Lost Age sees the group dynamic change, especially when Alex reveals himself as a Hidden Agenda Villain by the end, and potentially the whole series' Big Bad.