Ken

A teenager who is an expert in knife throwing. During his stay with Nijū-Mensō, his role during a job was usually more in the execution stage than the planning or preparation stage. He was the one who originally shortened "Chizuko" to "Chiko", a nickname which Chiko quickly picked up on. Chiko calls him Ken-nii-chan (lit. older brother Ken) as he appeared to be the youngest member of the band save Chiko, and the two were very close. He was the one who taught her to throw knives. (Source: Wikipedia)