 If you liked
X
|
 ...then you might like
Saint Seiya: The Lost Canvas - Meiou Shinwa
|
In both, the two main guys are enemies that were childhood best friends. The goal is the same: two sides, where the bad guy wishes the world destruction and the end of humanity, while the good guy wishes not just to save the world, but save his (current enemy) best friend. Also in both the "Bad guy" used to be a very gentle and kind person but it was possessed by evil, and tries to defeat his best friend. Also in both animes, the bad guy has a sister that was also a best friend of the two, and that possesses powers.