Sephiroth

Ahh, Sephiroth.  I must say he is one of The Best anime chars of all time.  I mean, just look at him!!  He's great, he's enigmatic, and he's so much more developed than most badguys, like oh, say; Ultimecia?  Yeah, there's a great evil-doer.  "I don't want to be around people anymore, so I'll just destroy everything so I can sit alone."  There's some real depth.  At least Sephiroth doesn't want to kill an entire universe, just one lil'planet. 

Anyway, he's the arch-nemesis of Cloud, which had spiraled from curiosity on Cloud's part, to a personal vendetta for...(anyone who hasn't played the game and doesn't know the following should avert their eyes):

KILLING AERITH!!!

<Achem>  Sorry.

(Awww, so cute!)

 So, now that Aerith is dead, Cloud continues to follow Sephiroth until the entirety of Jenova and Sephiroth's plot is unfurled, and it is up to our Spikey Haired Hero and Company to save the entire universe from being ruled by the One, and Only:

SEPHIROTH