Three things I love about this one:
1. Celeste is almost fooled into thinking that the impostor is Blue, despite the presence of Blue's corpse right behind her;
2. The Nice Poser is never seen again (except for one minor cameo that doesn't really count for reasons that will be obvious when I get there) or even explained;
3. The impostor uses the Nice Poser as a distraction despite the fact that the Nice Poser is behind him, meaning that if Celeste were looking at the Nice Poser, she'd still have the impostor in her field of vision.
