Did you know? 

Shintaro's (Brethren's) name became Shintaro after he disappeared in one attempt and returned with a different identity; but after that, the name stuck. 

During one of the books, Shintaro (still Brethren) was taken captive by The Dark Lord and didn't return for several years. When he came back, he had defeated his father and changed his identity from mental trauma of not wanting to remember who he once was. After that attempt flopped, the name simply stuck with him for the rest of the time, sometimes being shortened to Shin.