I forgot all about the University. Yes, he is a student there. 

Well, Sebastian is your character. Though it would make more sense that Sebastian created it given the power of the first potion. (The hunter used two potions. The more powerful would coat him in the illusion of the 'James Thornton' bank robber until he had no need for it. So it would have lasted a pretty long time. The second potion nullified the paralysis spell Matthew's grandparents would have cast on him, which I guess wouldn't have needed to be so powerful or created by Sebastian)

I'm guessing Sebastian would feel responsibility. He could have been tricked into creating the potion or it could have been stolen from someone else, the hunter was obviously pretty cunning and daring, but whether Sebastian ultimately feels responsible is up to you. 

It seems like it's already in the job description for Sebastian to help Matthew in [i]some[/i] way. But so far as guardianship is necessary I'm unsure. If we wanted to go down the route of Sebastian helping Matthew out with a place to stay etc, I'd have to forego Matthew receiving any real inheritance from his grandparents besides what he needs for university fees for the immediate future (They could have had large debts, not have paid off their mortgage..) It could be a good opportunity for character interaction though!