[Azure (Kagono Tori)] My Pure and Innocent Little Sister Became Friends With a Mesugaki {Mistvern}

His little sister's sweet-seeming friend knocks on the door and instantly reveals her true nature: a smug mesugaki who straddles him and sneers that no self-respecting adult could ever lose to a child. What follows is a one-on-one war of stamina, as the tiny temptress in thigh-highs rides him with impossible grip strength and back-to-back orgasms, her bravado crumbling into ahegao bliss even as she keeps taunting him between rounds. By the time he floods her undeveloped womb with a record-breaking creampie—shown in glistening x-ray detail—she's already claiming victory, and the afterword rewards readers with the same girl beaming innocently in a swimsuit.