She is the seventh witch.
Rika has the ability called "Amnesia". That ability can make a person lose all memories of the witches. This ability is activated without having Rika kiss that person but instead activates by having her placing both of her hands on the target for several seconds, however, the effect takes 24 hours to manifest. However, on a Witch Killer, it erases the memories of those who met the target aside from the Seventh Witch, other Witch Killers, and Spotters, and also because of being the seventh which she also has the ability to detect the six other witches.
She worked for the Student Council to prevent the greatest secret of the witches from being discovered. She is in love with Yamazaki.
