Rave, 666 Satan Recommendations
In terms of characters, both the main guy character has an anti-self that they have to fight, and a supporting main girl character has a surprising past. The ending fight and twist is quite similar, except 666Satan probably have a sadder ending (relatively, but it depends on how you read it). In both manga, the characters (no matter who you are) gets stronger... and converts the evils. In addition, they all hinted some sort of romance and comedy.
Oh. They both have strange pets. haha.
A bit of a warning though, rave has an art style that resembles One Piece-ish. If you
In both people fight using some objects who gift special abilities. Both the good guys and the bad organization are searching for a number of objects that, when joined, will produce the reborn of a mythical beast that could destroy the world, Satan and Endless, respectively.