Sakura Sakurakouji may seem like a frail and demure girl, but in reality she’s a powerful martial artist with an unmatched sense of justice. One day, on her way home from school, she witnesses a boy murdering several people with bright blue flames. That boy is Rei Ogami, a merciless assassin known as a "Code Breaker", and the latest student to join Sakura’s class! While Rei appears to be the perfect gentleman at school, in truth he lives by the principle of "an eye for an eye" and is hired to target and dispose of criminals who have escaped the law. With everyone oblivious to his true nature, only Sakura knows what he really is and his remorseless feelings towards what he does. But despite his icy heart and his threat to burn Sakura and everyone around her should she reveal his secret, the righteous teen can’t help but feel that there’s more to Rei than meets the eye...
in both manga the main characters start off as cruel and cold people but after being exposed to a series of events that will inevitably change their lives the audience finds out more about their background and the mysterious power dwelling inside each of them.
If you're into the whole girls with megaboobs battling thing, these titles are a good match (though the two aren't that similar in plot, they will definitely appeal to the same fans of this sort of idea/theme). They were drawn by the same person, so you'll recognize a similarity in characters, as well.
Both of these series are about a guy becoming bound to a very powerful demon/spirit and gaining great power in the process. While there isn't much plot similarity beyond this fact, both series "feel" very similar to one another.