My Clubmate's a Little Devil!

Ruru is seemingly a timid, calm girl, but she has a devilish side to her: she just loves to tease and make fun of her weak-willed senpai. Unfortunately for her, no matter how much effort she puts into it, she fails as a walking stereotype of a teasing underclassman. After all, she’s a klutz and all her devious schemes backfire. Until she discovers one activity in which she can dominate her senpai: sex. And once she discovered that weak point, she made sure to "exploit" it as much as possible.

(Source: FAKKU)