"You'd better watch your mouth, old man!"
Though a rookie member of the Ryudo Family, he's a well-respected person among locals. He's also a loyal ally to Kiryu, as he tries to protect Sunshine Orphanage from being demolished by Mine, and accompanied the kids when the orphanage was demolished. He's also good with kids, apparent from his friendship with the Sunshine Orphanage kids, especially Izumi.
He's depicted as a skilled carpenter, being able to construct a dog's house for Izumi's pet dog Mame. He also has a great appetite as he likes to take free samples of food.
He appears to have a troubled relationship with his father, who disapproves of his status as a yakuza.
Mikio has an obese build and, judging by his black eyebrows, has his hair dyed blond. He wears a purple polo shirt, purple pants and a pair of grey shoes. On the shirt is a logo with an anchor and the phrase "only a saint's world is hell."
Mikio hails from a family who is famous for owning a restaurant in Okinawa. According to Nakahara's accounts, Mikio had a tough childhood (presumably due to his strict father), as he told Kiryu that young Mikio had to run away from his father to socialize.
He was sent to the Sunshine Orphanage along with Rikiya to hand an eviction note to Kiryu and to watch the orphanage for a while. Initially, he was hostile to Kiryu, but later become one of his allies.
When Yoshitaka Mine and Tetsuo Tamashiro arrived with an army to demolish the Orphanage, Tetsuo shot both Mikio and Rikiya in the legs. That didn't stop Mikio from attempting to prevent one of Tetsuo's foot soldiers from destroying Mame's doghouse. He was severely injured and battered with a sledge preventing it from happening, yet still failed to save the doghouse. Mikio nearly died from the attack, but after being rushed to the hospital, he was able to pull through.