Athletic, intelligent and popular, Riku is depicted as a tomboy, who is more serious, and down-to-earth than her sister. Though more outspoken than Risa, Riku can be cute and Risa notes that her sister doesn't realize just how popular she is[reference needed]. Though the girls are different in personality, Riku is protective of her sister and they share a tight bond.
When Risa is kidnapped by Argentine, Riku falls into a coma.
Innocent, spoiled, and depicted as "girly-girl", Risa as the opposite of her twin sister Riku, though both show the same stubbornness and tenacity, particularly when it comes to people they care about. She is initially confused with Riku and Daisuke falls in love, and jealous, but realizes she was more afraid of losing her sister and her friend than wanting Daisuke for and becomes supportive of the relationship.
Though Riku initially appears to only see Daisuke as a friend, its soon revealed that she harbors romantic feelings for him, though he initially can only see Risa. After her sister rejects him, Riku begins to see him less as clumsy and unreliable, coming to see his strengths and admitting to the feelings she herself had been denying. While trying to sort out her feelings, she remembers that they had met as children when he'd helped her retrieve her precious teddy bear from over a fence once night. Eventually she admits her feelings, but initially is disbelieving when Daisuke tells her he feels the same. Realizing he is speaking the truth, they become a couple. In the anime adaptation, Riku is more uncertain of her relationship with Daisuke, temporarily hating that he calls both her and Risa "Miss Harada" and acting jealous towards Daisuke.
Riku intensely dislikes Dark, feeling he is nothing but a thief and a playboy, however when Risa is in danger and he promises he will bring her back, she begins to soften her stance towards him. She also sometimes compares him to Daisuke, though in the manga she does not yet know that Daisuke is Dark. In the anime adaptation, Riku does find out Daisuke is Dark after he grows wings in front of her, to save her from falling to her death.
Dark also steals her first kiss when he is forced to make an emergency landing on the Harada Residence's balcony, and this is his only way to stop Riku screaming for help. She thinks Dark stole her first kiss and resents him. Though she has the imagination that it was (correctly) Daisuke too and started haboring confuse feelings for him.
- He wouldn't notice if I got a crew cut, shaved my head bald, or grew an afro!
- Wha? Bu? Dar? What language is that?
- They said that looking under the same sky is the one you love.
- Mom, dad, Risa became a delinquent!
- In the anime, Riku doesn't wrack her brain about Daisuke being Dark like she does in the manga.
- Riku was the winner of the 2003 Anime Saimoe Tournament.