Gonna Be The Twin-Tail!! Vol. 1 (Light Novel)

Soji Mitsuka is a first-year high school student at Yogetsu Private Academy with a serious obsession for twintails. So much so that he knows of every girl in school who sports twintails and even ranks and classifies them. One day, he meets a mysterious new girl named Twoerle who happens to be from a different world. Suddenly, extradimensional monsters called Elemerian that traverse worlds seeking out human moe energy appear on Earth and claim dominion over all twintails on the planet. Twoerle entrusts Soji with Tail Gear, a type of armor that harnesses the power of twintails, and enlists him in the fight to stop the Eremerians. There’s only one catch: the armor transforms him into a warrior girl with twintails!
Notice: This book is written in Japanese.