He has a very timid personality, which makes it difficult to ask Sumire out on a date. He is typically seen tagging along with his sister, who has a more outgoing personality.
Riku has brown hair and light green eyes. He is usually seen wearing his school uniform.
Riku first appears in Chapter 3, when he is listening to his sister's phone call with Sumire. After the call Nene decides that they will snoop around to find out why Sumire is acting strangely. Later this snooping around leads to Riku being heartbroken when he and Nene misinterperet the reason for Sumire to move into Venus Vanguard, believing her to be in love with Lucia.