Height 179 cm Uzuki, also known as Slur, is a killer seeking to overthrow the JAA. Roughly around the same time Tarou Sakamoto's bounty was put up, Uzuki murdered a quarter of the assassin industry. The Order deemed him "X" as he leaves a large red 'X' at the scene of his crimes. In chapter 104

Spoiler

Database reveals Uzuki's full name and part of his past.

(Source: Sakamoto Days Wiki)