Suekichi Matsuo (松尾末吉, Matsuo Suekichi) nicknamed "Matsu the Dud" in the Viz translation, is a character that appears only in the manga. Suekichi is a friend of Yusuke's from primary school who took up boxing. He was often bullied in his youth (one time his pants and underwear were taken off, and his arms were tied to a broom, preventing him from covering himself). Yusuke got him out of trouble, but only for a price. He's a great boxer, but is afraid to harm his opponents, and freezes from the shock of prior bullyings before he took up boxing.
One time, when being bullied by a rival school member named Itou, Yusuke, a ghost at the time, took possession of his body and beat one of the bullies, scaring the others and Itou off after forcing him to give Matsuo some money that he had stolen before. Yusuke had to stay in his body until his time was up, so he got Suekichi's esteem up by challenging Takahashi, the leading boxer of the rival school & ally of the bullies Yusuke beat. Afraid to fight Takahashi, Suekichi gets scolded by Yusuke; due to that, he forces Yusuke out, finally stands up for himself, and beats Takahashi. Matsuo is last shown surprised that he won by himself and that Yusuke's spirit was gone.
- Matsuo has some physical resemblance (mainly in his face) to Gen Nakaoka, protagonist of the 1970s Hadashi no Gen manga series, though his thick eyebrows are very reminiscent to Rock Lee from the Naruto franchise.