He could hear his name being called but since he doesn't know the voice, Koutarou assumed that there was some other Koutarou around.

But apparently he is wrong, because the person looking for a Koutarou is coming up to him. Startled, Koutarou stares up at the person. Koutarou thought he had remembered most thing. He knows he had a few gaps still but he's been in the Library for a fair amount of time by now. "I-... I'm sorry, I suppose that I still might have a few memory issues, did we know each other well?" What did he write? But they didn't enter the book to find an entrapped soul. And freshly transmigrated souls don't have very clear memories.