Stu is a vampire. He was turned a few time ago and has his own vampire gang. Together they go to George and put him E in his drink. George drinks a few of it and gets high. So Stu and the others make fun of him. Mitchell however is furious. He stops them and says that he is happy that George drank only a very few of it, otherwise he might have died, as they put an overdose into it.
Since he didn't die in the books he must still be with Mitchell when Mitchell becomes the leader of the vampires in Bristol. He wears the dark sun glasses and is completely dressed in black when meeting George.