Ruri Himeyuri is the younger twin sister of Sango Himeyuri, living with her without parents present. Despite being one year younger than Takaaki Kouno, she shares the same class as Sango. Her surname, Himeyuri, combines the characters for "princess" (姫) and "lily" (百合), while her given name, Ruri (瑠璃), means "precious stone" or "lapis lazuli".

Physically, Ruri possesses long violet hair styled in downward-tilted twin buns and spiky bangs, complemented by violet tareme-shaped eyes. She stands 152 cm tall with a slim build and typically wears a sailor school uniform complete with thigh-high stockings, a miniskirt, and ribbon accessories.

Ruri struggles to express her true emotions, even towards Sango. She speaks with a strong Kansai dialect and often displays tsundere tendencies, masking vulnerability with outward hostility. She exhibits fierce possessiveness and protectiveness toward Sango, manifesting as intense jealousy and antagonism when Sango develops an attraction to Takaaki. Ruri frequently shows physical affection by hugging Sango around the waist.

Unlike Sango, Ruri lacks academic aptitude and cannot develop robots, but excels in cooking and manages household responsibilities. Her narrative focuses on her emotional struggles regarding Sango's growing independence. Initially hostile toward Takaaki, her character development shows a gradual softening as she navigates her sister's relationships. In *To Heart 2*, Ruri shares a joint storyline with Sango rather than having an independent route. She appears as a side character in *To Heart 2 Another Days*.