He also forms rivalries with characters Draco Malfoy , a classmate from an elitist wizard family , and the Potions teacher and head of Slytherin House , Severus Snape . He also forms a rivalry with characters Draco Malfoy , a classmate from an elitist wizarding family , and the cold , condescending Potions master , Severus Snape , Draco 's mentor and the head of Slytherin House .