As the eldest of the Moonlight Brothers, Mitsunobu envisioned himself as Housen's ruler and boss. He initially confronted King Joe for Housen's leadership, but after his loss to then-outsider Joe during his first year , Mitsunobu was forced to fall under his command. He would eventually warm up to Joe, however, and would end up swearing loyalty to him.
Mitsunobu is picked by Joe to represent the Housen seniors in the five-on-five Suzuran-Housen War in the Monster's Forest; however, while he was hoping to face-off against injured batch mate Zetton, Mitsunobu lost the luck of the draw and faced the younger Hanaki Guriko instead. Despite his initial complaints that fighters should be of the same year level, judge Nakajima Shinsuke forces Mitsunobu to suck it up and go on against Guriko - who promptly kicks the Housen senior. Mitsunobu lose that fight .
He has a tendency to have frightening nightmares about his losses, like his fights against King Joe (where he had the nightmares for more than four days)  and Guriko (where the mere sight of the Afro Demon was enough for him to faint on the spot). Younger sibling Tsukimoto Mitsumasa has stated that whispering Guriko's name will cause Mitsunobu to suddenly jump up wide-awake after his fainting spells, which are all explained by his low blood pressure.
After graduating from Housen, Mitsunobu was able to get a job at a taxi company, but was immediately targeted by Muroto Zenmei and the Naitou Twins, Naitou Ichigo and Naitou Ichie, during the Amachi Army's rise to power . The beating he received from the trio thus sparked the war between Housen and the Army.
- Mitsunobu, five underlings vs. King Joe - Loss 
- Mitsunobu, Mitsuyoshi, several underlings vs. Gunji - Won 
- Mitsunobu vs. Guriko - Lost 
- Mitsunobu vs. Muroto Zenmei, Naitou Ichigo, Naitou Ichie - Lost