Rika Nakamura, one of the twin sisters alongside Yuka Nakamura, becomes central to Yusuke Yamamoto's life through childhood events. Yusuke once saved Rika and her sister from an attacking dog, leading Rika to develop deep feelings for him and promise marriage when they grew up. This rescue anchors her lifelong affection and commitment toward Yusuke.

Years later, Rika and Yuka reenter Yusuke's life when his father, a gynecologist, invites them to live in their home to help Yusuke overcome his severe fear of women, rooted in separate childhood trauma. Limited space forces Yusuke to choose which sister stays, sparking a competitive dynamic as the twins vie for his attention and affection.

Personality-wise, Rika is more reserved and less overtly assertive than Yuka. Though both sisters actively attempt seduction through physical advances and personal intimacy to win Yusuke's favor, Rika remains notably shy about confessing her true feelings. Despite her quiet demeanor, she fully participates in efforts to break through his phobia, driven by enduring love from their shared past. Her reserved nature contrasts with Yuka's boldness, positioning Rika as the narrative's second heroine.

The story depicts Rika navigating this competitive yet intimate living situation, where the sisters' rivalry escalates through increasingly direct tactics to gain Yusuke's preference. Her background as a childhood friend and persistent, albeit shy, romantic pursuit remain consistent throughout her portrayal.