MIzunuma was formerly the second most bullied student in Koryou High School in Class 2-B second to Kenichi. In the anime, he was to be picked as Daimonji's opponent, but Kenichi stepped in instead. Inspired by this act of courage, Mizunuma later joins Shinpaku Alliance as one of its pawns, and learns Karate from Alan Subishi from the Kyuukai (Secluded Ogre Association) Dojo located in an abandoned building.
Mizunuma is tall with glasses and smooth hair, not unlike Tsutomu Tanaka's style. Like Kenichi before his fight with Odin, Mizunuma was commonly seen in his school uniform during school, and at his dojo in Karate robes.
Also like Kenichi, Mizunuma is a shy person who was frequently bullied (second to Kenichi) and stands up for his beliefs of righteousness by gaining strength and joining Shinpaku. In this, Mizunuma looks up to Kenichi as a role model. Unlike Kenichi, Mizunuma's sense of justice against enemies is not defensive, but offensive, as seen at one point when he attacked a middle school bully so badly that he caused serious injuries (and even hit Matsui in the face at one point).
His other role model is Alan Subishi, his master. When Alan was incapacitated by Yomi's Tirawit Kōkin, Mizunuma was overcome with feelings of revenge and was first to step into the ring during the DofD tournament to advance Shinpaku so that he could fight Yomi.
Formerly the second most bullied student in the school, MIzunuma becomes inspired by Kenichi's courageous actions and joins Shinpaku as one of its grunts. During the budding Shinpaku Alliance's fight against Loki and his troops, Mizunuma held his faith in Kenichi, saying that Kenichi wasn't someone who would stand by and watch his friends get beaten up. He and the rest of Shinpaku are saved by Kenichi, who bursts in upon hearing of his admirable quality, and soon after, Takeda and Ukita, who assist Kenichi and score a victory for Shinpaku.
Wanting to be strong like Kenichi, Mizunuma also joins a dojo to quickly become strong. However, he becomes more violent and learns some unscrupulous fighting moves, such as using chairs as weapons and poking enemies in the eyes. He gets visited by Kenichi and Niijima, and is stopped by Kenichi from landing a finishing blow with a chair. Upon seeing Alan fall, Mizunuma advises Kenichi to finish him with nunchuku, but Kenichi denies them. Mizunuma stands by in confusion of his beliefs, wondering why Kenichi isn't running against the overwhelming number of the dojo's students, who have gathered at Alan's command, and watches Kenichi take them on. This confusion is then replaced by utter fear, as Sakaki bursts in and easily defeats student after student. Upon being questioned by Sakaki regarding his strengths and beliefs, Mizunuma realizes that he really wanted the strength to stand by his beliefs. The "dojo challenge" ends with Mizunuma continuing to stay at Kyuukai, now reformed with the help of Kenichi and Sakaki.
During Yomi's dojo-hunting spree, Mizunuma walks into the dojo to see everyone in there beaten by Tirawit. Seeing Mizunuma, Tirawit attacks quickly, only to be blocked by Alan Subishi, who claims that Mizunuma "isn't an official disciple" and collapses into a coma, saving his last student.
Like the other members of Shinpaku, Mizunuma boards Tanimoto's ship to Despair Island for the DofD tournament. Against the sentiments of the other members of Shinpaku, he bravely steps in and faces off against his opponent, who overwhelms him and puts him in a headlock. MIzunuma drops his glasses and counters with a series of backwards elbow strikes, called the Backwards Machine Gun, surprising everyone. However, he is once again overwhelmed by his opponent, who attacks relentlessly, prompting Takeda to throw in a towel. However, the match continues at referee Diego Carlo's decision, since Mizunuma hasn't quit when he still was unwilling to admit defeat. Mizunuma manages to hit his enemy between the legs, but turned away and is attacked by his enemy relentlessly. Mizunuma falls clenching his fist, remembering Alan's words, and refusing to lose until finished off and carried out, suffering serious wounds. His defeat brings out the will of the other Shinpaku members, especially Takeda, who goes in and finishes his opponent. Even Niijima notes that even though Mizunuma lost, his loss was a greater contribution to the team than if it would've been a win.
- Backwards Machine Gun: Is a series of backwards elbow strikes. Mizunuma learned this Technique from his master, Alan Subishi and it took him a month to perfect it.
- Vs Member J (Loss)