Mucchi Muchi na Osananajimi ga Orenchi ni Sujitore shi ni Kita

I am an ordinary college student, and I have a childhood friend who is one year younger than me, whom I've always treated like a little sister. Her problem this time is that “she's gotten fat while preparing for her exams,” so I decided to help her lose weight with some proper exercise… (Source: Comic Alive Plus, translated)