[Natsume] The Day I Met You in Days Gone By

A youkai's prank causes Natsume's consciousness to become attached to the past of the person he subconsciously hates most, allowing him to witness that person's uncertain youth through his eyes. When Natsume returns to reality and meets Seiji Matoba, head of the Matoba clan, again, he begins to wonder: What happened back then to turn the boy who once looked at his shikigami with such tenderness into someone so ruthless and decisive?

I originally wanted to play with a gender-swap idea, but once I started writing seriously, I couldn't make it work. I'll save that premise for next time. That's all...