The fact that the Potions professor plays favorites isn't really a secret, since he makes only enough effort to conceal it that he can't be told off for it by his superiors.
But the students he favors are not necessarily the ones you might imagine -- that is to say, not necessarily Slytherin, not necessarily best in his class, and not necessarily possessing of what has come to be regarded as a typical Slytherin disposition (think Draco Malfoy, Tom Riddle, Severus Snape).
The common denominators are wit, intelligence, and passion.
Daine Sadgrove, for example, is a favorite because of her resourcefulness, and he is intrigued by the fact that she is clearly breaking rules but no one has ever been able to prove it. Iris McDougal is a scrappy little Ravenclaw who is awful at Potions, but she has a sharp wit, and was able to establish a rapport with him on that alone (which in itself he finds intriguing).
His favorites are very clearly favored... he almost never docks points or disciplines them, even for disruptions, and he would rather speak to them after class than punish them. The highest number of points he has ever taken from a favorite was 10.
Those he doesn't favor he treats as one would expect from a young teacher -- with as much fairness as he can. His temper is somewhat legendary, but he does usually make an effort to contain it. Students he is apathetic toward generally get docked between 5-20 points, depending on the infraction.
Those he actively dislikes he will punish for the smallest thing. If someone like Tiran McQuillan twitches suspiciously in his presence, it's over -- they get detention, lose points, are sometimes threatened with expulsion... the whole nine yards. And he doesn't half-ass his punishments. Detentions are notoriously brutal, and students he dislikes generally get docked between 10-100 points, depending on the infraction.
