Rika Kawada is Yura Onoda's younger sister, introduced at age 20. She is defined by her significant sexual experience and promiscuous behavior. Initially, she simultaneously juggles multiple relationships: one primary boyfriend, three designated "sex friends," and various casual encounters. Her confident, uninhibited approach to sexuality starkly contrasts with her sister's early inexperience. Rika frequently teases and provokes both Makoto and Yura Onoda about their sexual inexperience, actively pushing them to improve their intimate relationship. She offers the newlyweds unconventional advice. Despite her seemingly carefree attitude, Rika undergoes substantial personal development. Her longest-standing connection is with Taku Yamada, her "number-one boyfriend," though their relationship remains unstable and intermittent for much of the narrative. Rika eventually shifts toward commitment, marrying Katori, the son of Makoto Onoda's boss. However, this marriage rapidly deteriorates and ends in dissolution due to fundamental incompatibilities with her husband, marking a significant departure from her earlier nonchalant relationship patterns. Professionally identified as an officer in live-action adaptations, Rika maintains her familial connection to the Onoda dynamics through her sister. Her narrative function consistently positions her as a foil to the main couple's sexual naivety, embodying a liberated attitude toward sexuality while navigating her own complex romantic challenges.