A mysterious girl only Rio can hear. When Rio got attacked by an assassin, she taught him how to manipulate his Ode and use Spirit arts.

Spoiler

She's an upper-high class spirit whose been contracted to him since the beginning of the series. She takes on a humanoid form, which is very rare for spirits. She apparently has no memories of her time before meeting Rio. She also seems to know of his previous life as Haruto, as she refers to him by that when they're alone and/or with trusted friends