Ruby, a fairy descendant and Dia's younger sister, possesses a naturally timid and self-conscious personality, especially concerning her unique appearance. Unlike Dia, who consistently maintains an adult human form, Ruby often appears smaller due to incomplete control over her size transformations.

She assists Dia in her activities, including Dia's secret role as the tokusatsu-inspired superhero Scarlet Delta. During missions, Ruby can possess and control Dia's magical motorcycle, augmenting its capabilities. This vital role forces her to overcome her inherent shyness at crucial moments.

Her background connects her to the Numazu community where she lives. Ruby interacts with residents like Yohane, Hanamaru, and Chika, though her introverted nature causes her to avoid extended social contact. She belongs to the Kurosawa family lineage, sharing its fairy heritage with Dia.

Ruby's core traits of shyness and deep loyalty, particularly to Dia, remain consistent. Her development emphasizes a gradual increase in confidence through supporting roles, not dramatic change. Interactions with others reflect her kind yet reserved disposition.