Ruri Himeyuri, born June 16, is the younger twin sister of Sango Himeyuri. She shares the same class as Sango and is one year younger than the series' protagonist. Both sisters speak with an extremely strong Kansai dialect. Ruri possesses long violet hair typically styled in downward-tilting twin buns, distinguishing her from Sango, and has violet eyes. She stands 152 cm tall with measurements of 80-56-79 cm, maintaining a slim build and medium breasts.

Ruri displays fierce protectiveness and possessiveness toward Sango, insisting they only need each other and resisting any outside interference in their relationship. She becomes openly hostile toward the protagonist Takaaki when Sango shows romantic interest, viewing him as a threat. This jealousy manifests as treating Takaaki like a pervert, even when he helps Sango. Underneath her hostility lies a desire for Sango's happiness. Ruri struggles to openly express her true emotions, often masking her feelings from others, including Sango. Her personality combines tsundere traits with a pronounced sister complex, frequently shown through physical affection like hugging Sango around the waist.

Academically, Ruri is less capable than Sango. She lacks Sango's aptitude for robotics development and is generally described as not very bright. However, she excels in domestic skills, particularly cooking, surpassing Sango's abilities. The sisters live independently, managing their household without parental supervision.

Her character portrayal remains consistent across official media, with no significant development or evolution documented in spin-offs, OVAs, or other extensions. Her core traits—protectiveness toward Sango, hostility toward Takaaki, and exceptional cooking skills—persist unchanged. Her given name "Ruri" (瑠璃) means "precious stone, gem, or lapis lazuli," while her surname "Himeyuri" combines "princess" (姫) and "lily" (百合).