Seishirou Tsugumi

Tsugumi is a hitwoman who was raised and trained by Claude of the Beehive Gangsters.Tsugumi is originally introduced as a guy who transfers to Chitoge's class, and has orders to protect her from Raku, whom she thinks forced Chitoge into the relationship. She and Chitoge are childhood friends; she tries to keep a ten year old promise of becoming strong to protect Chitoge. However, after Raku treats her nicely, she begins to develop some feelings for him, even though she is too stubborn and embarrassed to admit it. In America, her assassin code name is "Black Tiger", who had a rivalry with "White Fang". She does well academically, having ranked next to Chitoge on a yearly exam.

(Source: Wikipedia)

Read more at http://myanimelist.net/character/58885/Seishirou_Tsugumi#V03sDFz2AZgtThJm.99