Appearance
Riku has straight dark blue hair and green eyes. He wears the standard male school uniform.
Personality
Riku seems to have a calm personality, usually speaking casually to other people and his class seems to trust him in talking with the people in other classes. He does dislike people like Kazuomi Housen who use violence to get what they want and has no problem picking a fight with him if he goes too far.
Plot
Riku is first introduced in Second Year Volume 1 walking up to Kiyotaka Ayanokoji with Sakurako Tsubaki asking Kiyotaka if he would partner up with Tsubaki because she had a C- in her OAA and was in risk not finding a partner up until now. Kiyotaka ended up refusing their offer saying that he already had a potential partner in mind. Tsubaki and Riku accepted that and left after Riku gave his phone number to Kiyotaka if anything happened.

Riku clashing with Hōsen
He represented Class 1-C in a meeting between students of the four first-year classes, in order to negotiate regarding group formation for the upcoming uninhabited island test. He clashed with Hōsen during their meeting and accused him of being the cause of Class 1-C student Hatano's expulsion.
He is in the same group as Hiroya Tokitō and Hosen during the school trip in 2nd Year Volume 11, and is quick to notice when Tokito and Hosen disappear. He finds Tokito being beaten by Ryuen while Hosen is also about to get involved. He steps in to help his senior against Ryuen, whilst simultaneously berating Hosen. Before things can escalate further, all parties scatter when other students start to gather to see what the commotion is about.
Abilities
Riku is shown to have high physical abilities as his OAA is rated an A for Physical Ability. When he grabbed Hōsen's wrist to stop him from lashing out at the others, Hōsen immediately knew that Riku was fairly strong.
Etymology
- The name Riku means "land" (陸).
- Riku's surname Utomiya means "eaves, roof, house, heaven" (宇) (u), "capital" (都) (to) and "temple, shrine, palace" (宮) (miya).
Trivia
- For the 2nd Year arc, When asked about his aspirations for the next year, he answered 'To make sure I can differentiate enemies from allies. If enemies manage to get inside, something which can't be undone might occur after all'.[1]