@Andromeda Like I said, I don't the exact details for every game. I'll tell you what I do know so far.

In VI, he reincarnates as Kefka, but due to a side effect from reincarnating, he has no memory of who he really is. When he was experimented on, he did indeed go crazy, but he possibly also remembered that he was the Emperor. This would explain why he sucked out the Warring Triad's power: He wanted to be a god again, unknowingly following his master plan to gain new powers.

In IX and I, he reincarnates as Garland, and his time loop plan not only came close to succeeding, but shows that Garland was the Emperor's most successful incarnation, as the events of XII are in reality the aftereffects of the resulting time paradox. This doesn't necessarily mean that XII never happened. Rather, it's more along the lines of "it happened, but at the same time, it didn't happen." By the end of I, there will be records alluding to Ivalice's war, but the details will never become clear. Seeing this result, the Emperor decided to try messing with time again, leading to the events of VIII. I don't know much about that game though, so I'm not sure how he falls into the formula this time around. Either way, by the end of VIII, he's become extremely weak, and is forced to regain his strength by using the Warring Triad, but since he couldn't as a god, he decides to do so as a human. Cue Kefka.

I know very little about XI and XIV, so I can't say for sure what he does in those games. Seeing how Realm Reborn hasn't been around too long, it's possible that the Emperor hasn't shown up yet at all. For example, in XI, until I know a little more, I'm assuming he becomes Eald'narche, who didn't appear until Rise of the Zilart.

As for VII, I'm actually in the middle of that game, so I'm not sure who the true villain would be. Sephiroth would be the obvious choice, but doesn't Jenova pull some strings from behind the scenes?