That Hiiro is from the future.
The reason Snow was an unknown character to him is because it was he himself that got Snow into the story, but he will only find out about it when that future becomes the present, of course.
So Hiiro has no idea about a story that Rei and Snow are aware about, and it also reveals a bit of Snow's trust on Hiiro (so long as she understands it's the good one, not the bad one.)
Snow herself is vaguely aware of it, she doesn't understand, but she understands when past Hiiro told her that "tomorrow's Hiiro" would be a completely different person.
There is an event in the future where Hiiro responds to Snow from a sentence he said in that moment, thus Snow came to learn that the Hiiro she first met was truly back again.
To Snow, there are 3 Hiiro's:
- The first one she learned is the one that saved her
- The bad one
- The first one again, but for some reason he doesn't know her, so it's confusing.