Yui Kanakura

奏倉 羽

Yui is a childhood friend of Raku as well as his current homeroom teacher. She is also the head of the Chinese mafia group, Char Siu. [Spoiler] Yui also possesses a supposed key to Raku's locket, making her the fourth girl that Raku allegedly made a promise to when he was younger. [/Spoiler] (Source: Nisekoi Wikia)