Ruka Sarashina is characterized as a cheerful, passionate, and emotionally expressive individual. She is honest about her feelings and often speaks her mind, which gives her a direct and assertive personality. However, these same traits can also come across as self-centered, possessive, and overly forceful. Her brash behavior is partly rooted in her medical condition and the resulting sense of alienation she experienced from her peers, which led her to develop a deep fixation on finding a romantic partner who could increase her heart rate.

Ruka’s impulsiveness frequently causes conflict, particularly with Kazuya Kinoshita and Chizuru Ichinose. Her intense determination can sometimes become overbearing, leading her to act stubbornly or resort to emotionally charged outbursts when things do not go her way. She has been known to make rash decisions in pursuit of what she wants, often without considering the broader consequences.

Despite her flaws, Ruka is not malicious. Beneath her clinginess and at times confrontational nature lies a genuine desire to be loved and accepted. She deeply wants Kazuya to be proud of her and to earn the approval of his family. In more serious situations, Ruka demonstrates the ability to be reflective and cooperative, even with those she typically sees as rivals. Kazuya himself notes that under different circumstances, many of the traits seen as troublesome in Ruka could be endearing or desirable in a romantic partner.

Ruka Sarashina is a petite teenage girl whose most recognizable features are her short black hair, typically tied with a ribbon most often blue and her vivid blue eyes. When her ribbon is removed, her hair takes on the appearance of a voluminous, layered bob. She is frequently seen wearing her school uniform, which consists of a white button-up shirt with a tie, a blue blazer, and a skirt that she typically wears slightly hiked up.

During rental dates, Ruka tends to dress in a more stylish and eye-catching fashion, often opting for turtleneck sweaters paired with patterned short skirts. Her casual outfits usually include sundresses, though she switches to long-sleeved clothing and leggings for more conservative occasions.

Despite her youthful appearance, Ruka has a notably mature figure. While she humbly admits that her body isn’t as developed as Chizuru’s, she is still described as having a sizable bust and long, slender legs. Ruka is widely considered attractive and often draws attention in public, frequently being compared to Chizuru in terms of appearance by male characters.

Ruka Sarashina was born with a rare medical condition, an abnormally slow heartbeat which has significantly impacted her life. Due to the risk of collapsing from physical exertion, she was often treated differently by others, leading to feelings of isolation. In her search for emotional stimulation that could help regulate her condition, Ruka decided to become a rental girlfriend in hopes of meeting someone who could naturally make her heart race.

For a time, none of her clients had any noticeable effect on her until a chance encounter with Kazuya Kinoshita, whose sincerity and emotional openness triggered a genuine response in her. Moved by his authenticity, Ruka gradually developed romantic feelings for him and eventually decided to quit her job as a rental girlfriend.

She then coerces Kazuya into becoming her “trial boyfriend,” threatening to expose Chizuru Ichinose’s identity and her connection to Kazuya if he refuses. Although manipulative, her intentions are rooted in her desire to be close to someone who makes her feel emotionally alive. Her involvement becomes a persistent complication in Kazuya’s life, particularly as she later begins working at the same part-time job as him.

The given name Ruka (瑠夏) is composed of the kanji ru (瑠), meaning “lapis lazuli,” and ka (夏), meaning “summer.” The name evokes a sense of brightness and elegance, aligning with her vivid personality and striking appearance.

Her surname, Sarashina (更科), combines sara (更), meaning “change” or “renewal,” and shina (科), meaning “department” or “section.”