魔法の鏡 (Mahou no Kagami)

Background
On the other side of the mirror, was a "world where everything was reversed". Opposite "destinies" that never should've crossed.

"Mahou no Kagami" tells the story of a sick girl (Rin) who lived alone in a tiny room in the attic. One day, reflected in an old mirror which lay in the room, a boy (Len) suddenly appeared. He introduced himself as a magician and as the two started getting closer, she was no longer alone. Len granted all of her wishes: he cured her disease so she could walk again, and made her a princess in a beautiful castle. Even then, Rin felt like something was missing. By the time she realises it's because she wanted Len by her side, with or without magic, the magic was fading and he had to leave. Before he says goodbye, it's revealed that on the other side of the mirror was a reverse world, so he and Rin should have never met, as they had opposite fates. Even after he's gone, Rin knows she'd always be waiting for him to come see her once more.

The sequel to "Mahou no Kagami", "鏡の魔法 (Kagami no Mahou)", is an album exclusive song. It is the story told from Len's point of view.

This song was first released on Kagamination and is featured in Yamashizuku's EndlessroLL and it has entered the Hall of Fame.