Gonna Be The Twin-Tail!!
Synopsis: "Soji Mitsuka is an ordinary high schooler with an extraordinary obsession. Twin tails are his fire, his air, his will to live! But when a mysterious woman from another world enlists his help to fight hideous monsters with various moe fetishes, Soji much transform into a powerful pigtailedgirl?
With the help of his childhood friend, the class president, and his endless devotion to pigtails, Soji and his crew form a twin-tailed threesome to take on the monsters who feed on the attribute power of humans. But Sojis new pigtailed powers come with a price; his surging popularity as the new heroin in town makes the twin-tailed trend explode, giving the monsters ample attribute powers to feed upon!"
NOTE: This is a Funimation title being distributed by Anime Limited in the UK