Houkago, Boku wa Kimi ni Naru

Akiba, a high school boy who is seriously into cross-dressing cosplay, devotes himself to emulating his "oshi." Then one day, while he was dressing up as his favorite character, all alone in the club room, he is discovered by Shibuya, a gorgeous gal that he has a crush on...? This is the start of a secret relationship between these two polar opposites! They will devote their youthful enthusiasm to a life of "oshi promotion"!

(Source: Shonen Sirius, translated)