There's so many twists that the author could've pulled. The fact that the romance itself is great just makes this manga all the more amazing. But honestly, the strongest theory here is they're all in a simulation and Kujirai isn't a human per se. Maybe she's dead in the real world and Kudo specifically asked for her to be created. Maybe this place is meant to let people relive their nostalgia or desires. The Lolita and her clothes addiction, the girl hanging out with Kujirai and her flawless body. Kudou and his memories of Kujirai.