He knew Toshio Ozaki and Seishin Muroi since they were kids. He was four years younger than them. He would often follow them around. As he was sleeping, he and Susumu were bitten by Nao and woke up. He heard Susumu call out to Nao. Later, Susumu went into cardiac arrest and died. Toshio tried to revive him, but to no avail. Later, Mikiyasu and Susumu were taken to Mizobe hospital. He eventually dies and doesn't rise and Toshio is infermod of his death by Taniguchi. He was the fourth Yasumori to die, and the nineteenth death overall.
Mikiyasu is often confused with Yasuyuki Ootsuka due to them having a similar appearance.