Gonna be the Twin-Tail!!

Ore, Twintail ni Narimasu. (俺、ツインテールになります.) is a Japanese light novel series written by Yume Mizusawa and illustrated by Ayumu Kasuga. Shogakukan has published five volumes under their Gagaga Bunko imprint since June 2012. An anime adaptation is in production.

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

Official

 * http://www.tbs.co.jp/anime/ore_twi/