We'd only been dating for three weeks.

Despite that, I was cheated on.

What's more, the person my girlfriend cheated with was my best friend.

She told me 'it's too soon' when I finally worked up the courage to kiss her, but she has no problem kissing *him*...

Betrayed by the girl I'd loved since childhood and my irreplaceable best friend, I'm left utterly heartbroken.

Just then, my best friend's sister appeared.

She took my hand as I wallowed in despair, and said:

"I'll be your affair partner, Senpai. Let's teach them the pain of having someone stolen from them."

※Also posted on Kakuyomu under the same title.

※Now available in print. To be released on June 2, 2025 (available from May 29 at select bookstores and e-bookstores).

※The title has been changed as follows for the book release:

『Senpai, Won't You Have an Affair With Me? ~My Childhood Friend Was Stolen by My Best Friend, So Now I Intend to Steal Back My Ex-Best Friend's Sister~』