Midori no Hibi

From VIZ:

Seiji Sawamura is the toughest seventeen-year-old in town, feared by all for his fighting prowess and his deadly “devil’s right hand.” But at heart, Seiji is a softy, and all he wants is an end to his seventeen-year history of being a lonely single guy. Unfortunately his tough-guy reputation only serves to decrease his popularity with the ladies. But what happens when one day, his right hand becomes a girl?!