Ranmaru Samejima earned the moniker "White Devil" for his prodigious speed and strength in national kendo competitions. Kendo, a family tradition, consumed his life before middle school, leading to intense training and social isolation. His solitary existence shifted upon meeting Kei Enjouji, who developed instant romantic feelings for him. Though initially annoyed and disgusted by Kei's advances, Ranmaru became impressed by Kei's raw kendo ability and invited him to train at his family dojo. There, they shared their first kiss, an event that deeply affected Ranmaru despite his reluctance to acknowledge it.

As high school kendo team captain, Ranmaru's relationship with Kei deepened, culminating in mutual confessions of love and physical intimacy. Witnessing Kei's devastation after his mother's death during their third year, Ranmaru vowed to protect him. That same day, he fulfilled this promise by shoving Kei away from a yakuza hitman's car, suffering severe injuries that left him paralyzed on his right side. This ended his competitive kendo career and plunged him into despair. Kei provided steadfast support throughout Ranmaru's arduous physical therapy, strengthening their bond. Ranmaru regained partial mobility, but the permanent impairment on his right side forced him to adapt his kendo practice and daily life.

Characterized by deep loyalty and intense protectiveness towards Kei, Ranmaru struggles to express his emotions verbally, having said "I love you" only once despite profound feelings. This reserve contrasts sharply with Kei's open affection. Ranmaru actively avoids yakuza matters due to the attack, yet external forces repeatedly draw him in. He maintains a complicated relationship with Kei's half-brother, Kai Sagano, who develops romantic feelings for him. After Kai rescues Ranmaru from a drugging and attempted rape by a professor, Kai himself rapes Ranmaru in his vulnerable state. Despite this violation, Ranmaru holds no grudge against Kai and actively advocates for reconciliation between the brothers.

Ranmaru's development shows a shift from viewing himself as a burden to Kei post-accident towards embracing their interdependence. He wears a wedding ring from Kei symbolizing their commitment. Later, in events like the OVA "Much Ado About Nothing," lingering insecurities surface when Ranmaru mistakenly suspects Kei of infidelity after finding lipstick on him, leading to a brief separation. His protectiveness manifests physically when provoked; he once incapacitated a host club employee insulting Kei, showcasing his combat skills despite his disability.

His background includes living with his younger sister Yuki and their grandfather, owner of a kendo dojo, after their parents' death. This environment cemented his early dedication to kendo. Post-accident, his focus expanded to preserving his relationship with Kei and navigating conflicts stemming from Kei's yakuza-connected family. Ranmaru encourages Kai to mend his relationship with his bodyguard, Masanori Araki, and supports Kai's efforts to reform his family's criminal enterprise into legitimate business.