Shinjiro is among Takashi's classmates, and was admitted by the author to be taken from his other manga, The Life and Times of an Idiot (Kaichou Baka Ichidai). He later on becomes best friends for life with Masked Takenouchi in the manga (at least for one multiple-chapter story), and friends with both Takashi and Akira.
Although Shinjiro seems to be a very tough person because of his mohawk and attitude during his introduction, he quickly turns out to have a friendly and silly side that shows even if he is a little too interested in destroying Akira's house. He does not even fight back after Takashi slaps him during Akira's birthday.
In the chapter titled "The Soul of my Head" (which was adapted in the 17th episode of the anime), Shinjiro reveals to Yutaka that his mohawk actually is a wig, hiding a slick hairstyle. The reason is slightly different in the manga and the English dub. In the manga, he says that he wanted to rebel against his high-achieving elder brothers and father, but was afraid of telling that to his family, but in the dub, he claims that he uses the wig to avoid being treated badly by the other delinquent students. In both editions, Takashi enters the café where the two are, and finds out the truth about the mohawk because Shinjiro and Yutaka panic and Yutaka accidentally puts the wig on his own head.
Apart from this maneuvre, Shinjiro's intelligence is unusually low even for a Cromartie student. For example, when he and the others decide to give the gorilla with the "banana hanging in the ceiling"-test, he does not know how to solve it.
Shinjiro is the character whose design changes the most between his forms. His female form Shinjibee is a brunette with pigtails (who still keeps the purple mohawk), and in the movie, he only has a naturally coloured ponytail and a moustache.