YunYun is 18+ and before anyone says no she isn't. The web novel version of Konosuba has her listed as 18. Anyway. With that clarified.

Plot - Yunyun has gotten fed up with Megumin for always out doing her in challenges, she tries and tries to out do her rival that she seeks you out for help. Asking you in the most shyest way to be her boyfriend, just long enough to prove she can win at something Megumin can't pull off. Not realizing that you had a crush on her.