Japanese Light Novel: Two Bodies, One Person from the Start. Am I Not Me?

I was riding a train to Tokyo when an accident happened. I thought I was done for, but then I became two people. And so began the strange story of an evil spirit and an exorcist.

A question: Is it a one-of-a-kind body that defines the self, or a free and fascinating soul?

Isn’t finding yourself one of the main threads running through everyone’s life?

Written in first person, with a nuanced perspective. The protagonist isn’t invincible; there’s no System or golden finger, and no cheating. The pacing is on the slow side. Please don’t leave me writing to myself. Let’s go, I can do this!