Kaz and Jesper were together when they were pulled through into the forest and he'd lost sight of Mat in the chaos. It filled him with active terror once he got there and figured out where they were, and instead of going to where others were gathering, he went searching the forest. For anyone, really, but let's be real, mostly for Mat.

He doesn't shout his lover's name because there are a lot of creatures in here who would only see that as a person to attack, so he has to be quick and smart about it. His powers aren't working so he can only pull his guns if necessary, and when he hears fuck, he knows it's necessary. A gunshot sounds out, loud for the forest, and the vine is shot through is an instant before it can fully drag Mat away.

He runs up a few seconds later having shot from a distance, but he doesn't need his powers for perfect aim. That's always been him. He would recognize that curly head anywhere. "Mat, thank fuck."