Gonna be the Twin-Tail!! (俺、ツインテールになります.), abbreviated OreTwi (俺ツイ), is a Japanese light novel series written by Yume Mizusawa and illustrated by Ayumu Kasuga. Shogakukan has published 22 volumes under their Gagaga Bunko imprint since June 2012.
The series follows an young male high school student with an extraordinary love for pig tails (or "twin tails") where he encountered a mysterious girl from another universe while monsters begin attacking the city, declaring they will drain the world's energy in the name of the Twintails. He would then transform him into a female Twintail warrior.
An anime adaptation produced by Production IMS aired in Japan from October 9 to December 25, 2014.
Plot
Soji Mitsuka, an ordinary high school boy, has an obsession for twintail hair. He meets a mysterious girl named Twoearle, who is from a parallel world. When monsters appear and invade Japan. They feed off "attribute strength", the spiritual energy of humans. Twoearle entrusted Soji with Tail Gear, imaginary armor initiated by powerful twintail attributes. This allows Soji to transform into a girl and twintail warrior (ツインテイルズ関係者) branded Tail Red.