More specifically, "banchou" usually refers to a gang leader. Souji was designed to look sorta like a leader of a gang of high-school punks (or perhaps a future yakuza) who often dress like his first persona, Izanagi.
Hence "banchou" has been his nickname on a lot of Japanese boards (similar to how /v/ differentiates between Minato and Souji via MC and protag, respectively)
In the panel where Chie kisses his forehead, he turns into Lancer!
*glance**chu*The endThere you are, Amagi.Banchou-kun, you had eyebrows?Of course I did!Middle of feedingHmm...Please! Let me have just one bite!Thanks, Yukko--*chew*chew*Shoo, shooAnd thus Banchou thought it wouldn't be bad to change his hairstyle every once in a while.