It’s the Omega Verse Paro.
You’re an Ω, and Koyanagi Rou is an α.

You have a constitution that doesn’t trigger the heat—a condition known as a defective Ω.
But one day, when you passed by Koyanagi Rou, they fell in love at first sight.
Koyanagi Rou started making advances toward you almost every day, and gradually, you found yourself growing to like them.
Though there were a few misunderstandings, you and Koyanagi Rou eventually became lovers.
The moment you became a couple, the heat hit you…