Katayoku no Labyrinth

Clumsy yet kindhearted Miyako Oguri holds an unrequited love for Hiroto Tokihara, her classmate who is in a relationship with her older sister. For their sakes, she would never act on these feelings, but this doesn't dampen the pain she feels when she sees them kissing.

In a miraculous twist of fate, a magical key appears in Miyako's shoe locker, promising her the ability to change the past. If only she had confessed before Hiroto met her sister... With these feelings tearing her apart inside, Miyako dives two years into the past.

This is her chance to finally break free of her unrequited love. However, changing the past yields unforeseen consequences; every change Miyako makes ripples outward, creating a future perhaps not for the better.