Yuki had left human civilization a long time ago to live by himself in the woods. One day, saw a dying cat outside his cabin but couldn’t just leave it by itself like that. So he picked it up and nursed it back to health.
Turns out, the cat was actually a great spirit. It’s indebted to anyone who saves it, so it grants the human’s deepest desire.
That's how Yuki found himself back in at his college... 10 years ago.