I've been looking all night but I can't seem to find it.

This guy gets a female stalker and asks some guy he knows to be a pretend lover. Eventually the two become real lovers. And we find out that the stalker was the guy's sister or cousin.

They have a scene where they kiss by the window to prove to the female stalker that they really are dating.

Thank you~

There's an old saying my late grandma taught me. When you're ten, they call you prodigy. When you're fifteen, they call you a genius. Once you hit twenty, you're just an ordinary person. About three years until I'm ordinary. Man... I can't wait to be ordinary.

I believe it's a oneshot from Priceless Honey, Fine Play. 😃

Thank you!!! I dunno why i couldn't find it. I looked through tags that the manga was tagged under. But shrugs. Thank you for your help!!

There's an old saying my late grandma taught me. When you're ten, they call you prodigy. When you're fifteen, they call you a genius. Once you hit twenty, you're just an ordinary person. About three years until I'm ordinary. Man... I can't wait to be ordinary.