Menou Kurashina is a minor character in the anime.  She is an young ill girl who is locked up in her room.  One day, however, she happens to meet the main characters grandfather while he was returning home from stealing.  Even though, she seems to play an insignificant role in the main series.  Her character plays a major role in Daiki's life.  She befriends him, despite him being a thief and in the end, despite her premature death.  Daiki says that he believes that she is the one who had come to save him.  She is a shy, timid girl who plays an important role in the story due to her connection to Daiki's life.  

Niwa Daisuke is your average teenager, until his fourteenth birthday. He tries to declare his love to the girl of his dreams, but fails. Suddenly, whenever he thinks about this girl he turns into the legendary phantom thief known as Dark Mousy. Daisuke's mother, very much aware of this, makes him turn into Dark and steal valuable works of art for a purpose unknown to Daisuke. As the story unfolds, he learns why, and about his classmate who seems to have the same ability.
 
(Source: Anime News Network)

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Daisuke Niwa is an ordinary, if slightly unlucky, middle school student. On his fourteenth birthday, he comes down with a 'condition' that has plagued the men in his family for three hundred years. When he sees his crush, Risa, he transforms into his alter ego, the phantom thief Dark Mousy. Unfortunately, when Dark Mousy sees his crush, Risa's twin Riku, he transforms back into Daisuke. The only cure for this craziness is if Daisuke can get Risa to fall in love with him ... 

(Source: Tokyopop) 

Included one-shots: 
Volume 1: Namaiki no 'N' (N is for Nishiki) 
Volume 2: OumagaAGAIN (The Demon Returns) 
Volume 4: Kanno-ke wa Kanai Anzen. (Protecting The Kanno House) 
Note: Includes 5 extra chapters.