Rumi Inuyama hails from the Inuyama family, who moved from Kyushu to Tokyo's upscale Roppongi Hills with their pet dog, Tetsunoshin. Her father, Masao Inuyama, runs an IT company strained by their costly lifestyle and Roppongi Hills expenses, threatening bankruptcy. This financial pressure indirectly involves Rumi when Tetsunoshin learns of it and works to help.

Rumi has long brown hair and brown eyes. She typically wears a blue shirt with a pink jacket and pink skirt. Personality-wise, she appears sweet and innocent but initially tends toward financial irresponsibility, freely spending on things like clothes. However, discovering Tetsunoshin's efforts to support the family prompts growth; she shows maturity by returning purchased clothes, indicating a shift toward fiscal awareness.

Her central relationship is with Tetsunoshin. She deeply cares for him as his primary owner but physically disciplines him, like hitting, when he misbehaves, as shown in episode 19. A key development occurs in episode 40 when Rumi discovers Tetsunoshin and his human alter ego, Celebrity Knight (created by Seto's magic), are the same. This revelation leads Seto to transform her into a dog in episode 41, enabling her to visit the dogs' secret dimension, Dog Paradise. She later returns to human form.

Rumi's family includes parents Masao and Yoko Inuyama, and grandparents Toranosuke and Kaoruko Inuyama. She attends Saint Lady Academy.