Mostly because the locals feel...off. Most of the time they are mean, outright cruel people. They'll probably insult you for being Japanese, of nothing else.

[Typically they throw that, accusations of witchcraft, or homophobic slurs at Rei. How they know she has no clue.]

Dreamers are also typically more alert seeming. And tend to stand out, some. You'll know. Trust me.

[And don't dwell on the guilt. For now, just take her seriously when she says she's missed you.]

I'm sorry that it's vague and confusing. I know this is totally new ground.