Watashi, Nibanme no Kanojo de Ii kara

"I also like you the second best, Kirishima-kun" Even though Hayasaka-san and I both have someone else that we like better, we are dating each other instead. Now, it's certainly true that me and Hayasaka-san are lovers. We go home together, get together in secret, and do naughty unspeakable things. But second-choice is still only second-best, so if we ever have a chance with our true loves, then this relationship will end. That was our agreement. "I'm sorry. I'm an idiot, and I keep falling for you more and more." Even though we both get close to the people we like the most, we keep on getting deeper into each other. We couldn't stop, and we couldn't just let go, no matter the consequences... The result of this hopeless, 100% dangerous, impure, unhealthy, and complicated love affair will be... (Source: Kadokawa, translated)