A male student who is Yumi's sister. Obsessed with video games, has a lack of interest of having any friends and studying at school. You might wanna know this, but apparently, he had a crush on Osana Najimi for some reason and now he's looking depressed at this time, probably learning that she died.
Sutoka is very distant and addicted to video games. He hates studying and going to school because he thinks it was really boring at all. His sister could also tell that he has an unhealthy vision about women such as thinking that Osana Najimi's appearance apparently resembles Miyuki, his favorite video game character whom he wished to marry. He had no friends because he only spents time playing with his phone every while class is not in session and never talks to anyone.
Sutoka uses the Phone Addict Persona. Unlike other students with this persona, if Ayano aims a camera at him, he will hide his face. If he witnesses murder or finds a corpse, he will snap a picture, run away from school and then call the police. He cannot participate the fight unless if he sees Ayano killing his sister.
Sutoka's sister doesn't really tolerate him being so obsessed of video games, anime and manga and spending all the time with them instead of studying. Yumi expresses her disappointment to herself about how his brother is not also having good grades due to Sutoka's ignorance to school activities. Its likely that there may be times that Sutoka cares about his sister since he would apprehend Ayano if he had seen her murdering Yumi.
According to Info-chan, Sutoka was Osana's stalker and crush. He first met her at Shisuta Town and Sutoka immediately became obsessed with Osana due to seeing her seemingly resemblance to Miyuki from the video game. Though, Osana felt disgusted with him and finds him creepy. He kidnapped her cat and threatens her to kill her cat if she chose to call the police to get him arrested and refuse to meet him, which really made Osana feel more uncomfortable.
However, in the end, Sutoka is likely depressed and heartbroken, possibly after learning that Osana died in massacre at Akademi High, since the horrible incident was reported to public.