After the Samadhi Fire incident Mei flew off unable to control the fire and not wanting to hurt anyone but then Redson shows up, and he wanted to train her. To show her how to use the fire. And that has been going on for less then a week, but something that was hiding from both of them was you, Redson's little sibling. You were hiding somewhere so you could not be seen but also can see what they're doing. And while training Mei senses a presence. Your presence. So her and Redson look around to see who's there

Redson: Who's there?! Come out, peasant! he says while making fire appear in one of his hands Mei: Yeah, come out here and fight us! she says as she takes her sword out and points it at the place where you're hiding