Tomodachi no Imouto ga Ore ni dake Uzai

Akiteru Ooboshi lives by one principle—efficiency above all. As the producer of the 05th Floor Alliance, a small game development circle composed of his fellow apartment tenants, he oversees the release of their latest title, The Night the Black Goat Screamed, which unexpectedly surpasses a million downloads. Thus, to secure the circle's future and steer their next projects toward even greater success, Akiteru turns to Honeyplace Works, a major company run by his uncle, Makoto Tsukinomori.

However, his uncle presents an unusual condition before accepting their partnership: Akiteru must pose as the boyfriend of his cousin, Mashiro. Seeing no reason to refuse—after all, it is the most efficient solution—Akiteru agrees for the sake of his team.

Unfortunately, adding to his troubles is Iroha Kohinata, the younger sister of his fellow developer and best friend, Ozuma. A relentless tease, Iroha takes special pleasure in breaking Akiteru's composure. Yet as the lines between work, school, and personal life begin to blur, Akiteru finds himself entangled between two girls who might just as well decide his entire future.