Keikenzumi na Kimi to, Keiken Zero na Ore ga, Otsukiai suru Hanashi.

Ryuuto Kashima is a gloomy high-schooler who quietly spends his days watching his crush, the beautiful Runa Shirakawa, from afar. She rules the school and has had no shortage of boyfriends. Although Ryuuto is fully convinced she'd never accept him, fate leads him to confess his feelings to her anyway. To his astonishment, she accepts—simply because she's "available" at the time—but the surprises don't end there. She immediately invites him over as if it's the most natural thing in the world! This odd couple is different in just about every way—the friends they keep, their hobbies, their love lives—but with every passing day, they grow a little bit closer. This may not be Runa's first relationship, but their dating story is going to be full of firsts for them both!

(Source: J-Novel Club)