Gonna be the Twin-Tail!!

Sōji is an ordinary high school student with an extraordinary love for pig tails (or "twin tails"). One day a mysterious girl from another universe named Thouars appears in front of him. At the exact same time, monsters begin attacking the city where Sōji lives, declaring they will drain the world's energy in the name of the Twintails. Sōji receives a powerful armor from Thouars called "Tail Gear" which transforms him into a female Twintail warrior named Tail Red.

Synopsis from Anime News Network