" But in addition, in Dumbledore’s absence, Snape has assumed unofficial duties as deputy headmaster, supporting McGonagall in doing what they can to protect everyone covertly from Umbridge’s rule. Running toward Trelawney’s screams, tending to Montague, training Harry to shut out Voldemort: these all come from the same urge within Snape to defend against the Dark Arts. He goes where he is needed. "

― Lorrie Kim , Snape: A Definitive Reading