Touka Akatsuki (朱月 藤果, Akatsuki Tōka) is the main antagonist of Gaim Gaiden: Kamen Rider Zangetsu. She was a servant of the Kureshima family and a childhood friend of Takatora Kureshima who transformed into Kamen Rider Idunn (仮面ライダーイドゥン, Kamen Raidā Idun).

Touka was among the list of children in Zawame Children's Nursery where they were part of a lethal experiment led by Amagi Kureshima that allowed them to control Helheim at will. In her teenage years, she was hired by the Kureshimas as a maid servant. She became close friends with Takatora during her time as a maid/servant, which she baked an apple pie for her close friend. Years later, Touka reunites with Takatora and she spends her time with the Kureshima brothers for a short while, however, she tells the Kureshima brothers that their father had passed away. Unknown to them, she gained a Sengoku Driver and a Ringo Lockseed to become Kamen Rider Idunn. She reveals that she is the one responsible for the murder of Amagi Kureshima and later she would attempt to murder the Kureshima brothers and destroy Yggdrasill facilities along with it, as an act of revenge for turning her into a guinea pig. However, she never realized about her time with Takatora and Mitsuzane, of them treating her kindly as a human being, and seeing her like a sister to them.

As Kamen Rider Idunn, she is responsible for the attacks on the Kurokage Troopers, Sid and almost attacked Yoko. She uses the ability to control the flow of the Cracks due to the Ringo Lockseed's effect.