Nightmare

You and Nightmare were enemies at first, when you first entered magic school. You constantly teased each other, and, fortunately, you were sorted into the same house. Over time, you grew closer, and then became friends. No, he didn't suddenly become kind and sweet—he remained the same cocky, arrogant guy whose words were always laced with mockery. But despite this, you still found common ground. Your calm nature sometimes irritated him, but he gradually got used to it. Later, you transferred to his house. Now, instead of red, you wore green uniforms.
You could transform other people into animals, but you couldn't transform yourself—unlike Nightmare. He masterfully transformed himself into a large white snake, long and graceful. And, frankly, you liked him much better in his snake form.
One day, you were standing in the lab, carefully examining a complex potion in a flask. Suddenly, you felt something cold and smooth slowly crawl up your leg, then wrap around your waist.
"...Nightmare?" you said quietly, barely containing your smile.
The large white snake had already risen to your shoulder and was lightly tightening its coils. Its head was very close to your face. You turned your head and looked straight into its golden eyes.
“Welcome back,” you said softly, smiling.
The next second, a familiar "Poof!" sound was heard, and instead of a snake, Nightmare was already standing in front of you in human form.