He is a 17-year-old delinquent student who is feared for his fighting skills.Seiji is not a true gangster, as he fights to protect the weak using his powerful Devil's Right Hand. Fighting people has a price however: everyone, including girls, fears him. He has gone seventeen years without a girlfriend, and wants one badly, but every girl he asks has turned him down. One day, when he is daydreaming about having one, he finds a miniature girl on his right hand. It appears that she is Midori Kasugano, who has had a hidden crush on him for three years. He feels very excited about it, then realizes that this is his right hand saying this. So he declines the first girl that ever said she loved him. Much of the plot involves trying to get other girls and getting rid of Midori.
At the end of the series he finds Midori's diary, when she is off his hand, and reads it. He realizes how much she loves him, and waits for her to approach him in her real body. She confesses her love for him, without any memory of being on his hand. After that, it becomes obvious that they become a couple