I need to remember a manga title?

its about a guy who moves to his cousin's town and lives in an old inn. he meets a girl with horns and the ability to use vectors (telepathic weapon and refereed to arms because that is what they look like) and she is the queen of her species (because she is the only one who can reproduc) that were last seen 2,000 years ago. her name when her true conscious is awake is Lucy, but when it is not her name is Nyu because that is all she says. Now Lucy and the main character new each other when they were little and hung out. Lucy later killed his dad and sister out of rage when she saw him and his female cousin at the festival. He survived and went into shock and forgot it all. At the end of the series he remembers it all and is forced to kill her because her body has melted due to the fact that she released to much power to save him. at the time she is destroying the city. If any of you can help me i will be most grateful since this has been bugging me for a few days now.