Reze

Reze does not actually have a speaking role until the post credits scene of episode 12, but the show puts her there on purpose. She is working a coffee shop near the school Makima keeps pointing Denji toward, and the camera lingers on her long enough that a first time viewer will remember the face.

Within the larger story she is a Soviet assassin sent to retrieve the Chainsaw Devil's heart. She is a Bomb Devil hybrid, which means she has the same kind of contract Denji has and the same kind of leash, just pointed in a different direction. Season 2 will use her as the next test of what Denji actually wants out of his life, and the fact that her first appearance is a gentle smile in a coffee shop is the entire joke. Everyone in Chainsaw Man who seems kind is about to become a problem.