Ore, Twintail ni Narimasu. π

Souji Mitsuka is an ordinary high school boy who has an obsession for twintails. One day he encounters a mysterious girl named Twoearle, who is from a parallel world, when monsters appear in his town and declare that all twintails in the world belong to them. These monsters feed off "attribute strength," the spiritual energy of humans. Twoearle entrusted Souji with Tail Gear, imaginary armor initiated by powerful twintail attributes. With the armor, Souji transforms into a girl and twintail warrior branded Tail Red, to protect Earth.

(Source: MangaDex)