Sada Mizuki is a first year student at Ishigami High School and a member of its sumo club.
Since a young age he was good at any sports and enjoyed them all.
Sat in attendance for the National Elementary Sumo Championship. After witnessing Onimaru, Sada became interested in sumo. Upon joining sumo he found success in becoming the Middle School Yokozuna.
District Preliminary arc
When Ushio and Yuuma enter the Ishigami High dojo, Sada is being reprimanded by his captain for attempting to skip practice, causing the young man to complain that this is why nobody wants to join their sumo club, which angers his superior even further. Before anything drastic can occur, however, Sada points out the presence of the two Oodachi High students. He then attempts to convince his captain to let the two practice with them, with no success. When Ushio takes off his shirt, Sada comments on how his amount of muscle directly contrasts his outer appearance.
After a hour of training, Sada teases his captain about being shocked by Ushio's stance, saying it must have been attained by constant training. He then asks the short sumo wrestler his name, and is shocked by the answer. He then chuckles when Ushio says that all of the things he did as "Onimaru" are ancient history. When Kanamori hesitates to combat Ushio, Sada encourages the pair of them, stating the match should be interesting. He then goes on to say that Ushio is fighting people in the wrong order, and that he should fight "the first year ace of Ishi High" first, albeit without the self-imposed punishment of quitting sumo if he loses. Before he can say anything further, Sada is unceremoniously grabbed by his captain and tossed near Yuuma. He comments on the seriousness of his captain, and explains his superior's fighting style to the Oodachi High sumo wrestler when the latter asks if he is strong. When Yuuma questions Ushio's reasoning for fighting somebody that physically outstrips him in constitution, Sada merely replies that they will see what skills Onimaru has to show them, and muses whether it is just a remnant of the past or not.
- Sada Mizuki vs Ushio Hinomaru