Watashi, Nibanme no Kanojo de Ii kara.

Shirou Kirishima and Akane Hayasaka have a secret relationship that they keep to themselves. Despite having feelings for other people, the two act like lovers when they're alone, knowing their hearts ache for their first choices. They even made a promise to end things the moment feelings with their first choice became mutual. But as they grow closer with each other and their crushes, the act becomes something real, something unexpected. When it comes down to picking their crush or the person that has grown closest to them... will they be able to make the right choice?

(Source: Seven Seas Entertainment)