Youko Nanase (七瀬 陽子, Nanase Yōko), real name Youko Chigi, is the mother of Takeshi Nanase and Gekkou Nanase who treats Takeshi coldly after the incident that involved both her sons. It is later revealed that she was one of the legendary 15 magicians from the war 16 years ago where she was the original wielder of Twilight, but she lost her magic due to a certain incident and her connection to the Ghost Trailer's leader.
She trains Takeshi in Episode 10 and Episode 11 and she seems to know something about the accident Takeshi doesn't. It was later revealed that she requested Gekkou to be killed in the 'accident' which failed in the end.The reason was that she saw Gekkou turning into Kazuma Ryuusenji numerous times in her dreams so she tries to prevent it by requesting Gekkou to be killed in the 'accident' which failed.
Gekkou tried to kill her so she can tell him the reason why she tried to kill him then after the fight that Gekkou and Takeshi had, Kazuma saves her life.
- The name Youko means "light, sun, male" (陽) (you) and "child" (子) (ko).
- Youko's surname Nanase means "seven" (七) (nana) and "swift current, rapids" (瀬) (se).