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~

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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!!

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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.