Iwaya Masaru

Masaru is childhood friends with Rin who soon becomes his stepsister after his father remarried to Rin's mother Sumika. His father dies in a traffic accident soon after the marriage, thus leaving Sumika to raise Rin and Masaru alone. He chooses to call Sumika by her first name, rather than calling her "mother". He also does not have good grades due to the lack of motivation.