Shiki Tsukai

Young Akira has a destiny: He's a shiki tsukai, a warrior with the magical power to control the seasons. He also meets the beautiful Koyomi, another warrior, who is sworn to protect him. For there are evil forces intent on destroying Akira - and the entire universe!

This manga contains these tropes:

 * Emotionless Girl: Koyomi Sakuragi
 * Familiars: The Kijyuus of the Shiki Tsukais.
 * Fan Service: Koyomi's clothing vanishes when she transforms, and she is not pat all shy about being seen in her underwear.
 * Katanas Are Just Better: Koyomi Sakuragi can turn into a katana.
 * Equippable Ally: Koyomi Sakuragi through Voluntary Shapeshifting.
 * Older Than They Look: Akira's Hot Mom, Megumi Kizuki, is sometimes mistaken for his sister.
 * Thanks for The Mammary: In order for Koyomi to transform into a katana, Akira must touch her symbol which is on her chest.
 * Well Intentioned Extremist: Some Shiki Tsukais say they want to save the Earth by killing the majority of humans.