Sour and Sweet

William Astor, a werewolf researcher, is excited when he gets the job of DADA professor at Hogwarts. He wants to teach, but also study, namely Remus Lupin. What he did not expect was just how interesting he would find the boy. Sirius is extremely unhappy about the friendly relationship, and takes it out on people around him. Teacher/student, eventual moment of S/R. Warnings inside.