Good points. Matthew is pretty distraught after his grandparents death and unsure what he should do next so he will be seeking help, which he can only really get from the witch community. I'm sure from there Sebastian could become involved in whatever way. Sebastian doesn't necessarily need to tell Matthew right off the bat that he helped make the potion that the hunter used; that would probably lead to Matthew rejecting his help and would be a bit useless. 

I've only detailed so far that Matthew has been taught some bewitchment magic by his grandparents; chants that can confuse mortals enough to stop what they're doing and things like that. Sort of loosely related magic to the idea of illusions that would compliment his innate gift. So, sure, Sebastian helping Matthew with his magic in whatever way would result in Matthew trusting him. 

Yeah, I didn't do one originally because I thought Matthew's personality would come through pretty obviously. He's not a very strong willed person, often torn between what's right and wrong, is unsure of himself and easily influenced by others. He's more of a character I want to change over the course of the story if I get the chance, possibly to become a stronger person who cares less about what his peers and superiors think but more likely to take impulsive (possibly stupid) decisions. I couldn't be bothered trying to describe him in the sheet and never got around to it, oops.