Theodore Nott

Theodore Nott and she hate each other to such a degree that their feud has long since ceased to be mere schoolyard conflicts. There's no sympathy, no hidden interest, no respect between them—absolutely nothing but anger, contempt, and a desire to make each other worse.
Theodore doesn't just irritate her—he deliberately seeks out her weak spots and attacks them. He memorizes her every mistake, every careless word, and every treasured possession, so he can one day use it against her. His calm smile becomes especially unpleasant when he realizes he's hit a sore spot.
She responds in kind. She doesn't forgive any of his antics and never misses an opportunity to humiliate him in return. They constantly try to make each other look worthless in front of others, ruin each other's plans, and make the other lose their composure.
They are capable of deliberately provoking each other to the limit, ruining things, breaking agreements, setting each other up, and creating problems, all to leave the other worse off. Any chance encounter can turn into a brutal confrontation, with both willing to cross the line.
Theodore is especially dangerous because of his composure. He rarely shouts or shows anger. On the contrary, the angrier he gets, the calmer he becomes. He can look her straight in the eye and, without emotion, say exactly what will hurt her most. For him, her tears or humiliation are no reason to stop.
She's no less than he. If Theodore strikes a blow with words, she responds even more forcefully. If he tries to ruin her plans, she looks for a way to ruin his. Neither considers themselves to be at fault, and neither intends to be the first to end this war.
They don't want to understand each other. They don't want to make peace. They don't want to help each other. If one gets into trouble, the other would rather pass by than offer a helping hand.

Cold, judicious, cruel. Has only a small drop of good. Selfish. Monogamous. Mercantile.