Alice, Yuki, Lorelei and Luize are all descendants of four gods that created them with magical season powers to protect a land that was separated from Japan by the gods, called Senkaihi, which was being attacked by demons and evil youkai, and was being the main source of monster apparitions that started to cause disaster on Earth. However, the fifth, traitor god created the fifth girl Yamiko, to destroy Senkaihi instead of protecting it.
Yuki is the master of the Winter season. She's extremely calm and shy, but also a caring girl. She is able to manipulate cold air, and despises anything that is hot, which causes her to sometimes argue and even battle with Luize.
Luize is the master of the Summer season. Luize is a very happy and energetic girl. She tends to be scandalous but sometimes she shows that she's very smart when helping the other heroines during the quest.
Momos are an animal-like species that mostly live as lovely and caring partners or companions. Momos are very loyal and dutiful, and most of them look like different animals, the most common ones being dogs and cats. The main Momos in the series are the heroines' pets: Hiroshi, Mai, Ka-ku and Tae.
Yamiko is the master of Darkness. She was created by one god that was actually a traitor to the other four gods who created the other girls. She was made to manipulate "regular magic powers" that ended up being dark magic. Her creation was to actually destroy Senkaihi instead of protecting it, making her the main villain of the series.
The series is highly inspired by the Touhou Project series, created by Team Shanghai Alice / ZUN, sharing a similar take on Japanese mythology by featuring youkais and other mythologic entities and/or species.