All of Slytherin House
The third, fourth, fifth, and so on until you've lost count, people who call you a Mudblood are almost all Slytherins. There's a particularly snobbish Ravenclaw and a priggish, awkward Hufflepuff who begs your pardon at once. But by the end of your first year, you're used to hearing it from the Slytherins.
You can never quite bring yourself to say anything too terrible back, but usually you don't need to. The four rowdy boys from the train always do it for you, when they're around, frequently with hexes. It surprises you how gallant they can be, since they also take every opportunity possible to tease and scold you about your relationship with Sev, and Potter and Black have taken to hiding black beetles in your hair during Defense Against the Dark Arts. You've tried reporting them, but detentions seem to be the breath of life to those two.
Sev tries to defend you from his Housemates, but the more he does so the more he suffers. Already the other Slytherins taunt him for his Muggle father.
Your second year, Lucius Malfoy has graduated, something you can only view as a blessing. He always looks at you like you're a bug, and he has far too much influence over Sev. For some reason he made Sev his protégé and saved him from hordes of other blood-conscious, conceited Slytherins, and now your best friend won't hear a word against him.
Also, Narcissa Black's scowls and perpetual bad mood are explained: you discover her sister Andromeda ran off with Muggle-born Ted Tonks, thereby "disgracing the noble name she bore." Good for her, you think, because you're young and you still believe that love conquers all.
The summer before your third year you and Tuney don't say a single word to each other. You hate it.
You start new classes, Ancient Runes and Arithmancy with Sev, Care of Magical Creatures by yourself. Divination scares you, and you already know more Muggle Studies than the professor, most likely.
Slughorn calls you a natural in Potions. You sit with Sev in the exact center between the Gryffindors on one side and the Slytherins on the other. At first you feel conspicuous, but eventually you get to be comfortable there.
You love Ancient Runes, but Arithmancy is hard. The only other person in your dorm who seems to understand it is Remus Lupin. He is quiet and studious, in spite of his best friends, and he never calls you a Mudblood or asks you out on a date. You think of him as a very safe study partner.
It's during your fourth year that James Potter first asks you out. He doesn't give up easily, either.
"Potter and the Mudblood?" sneer the Slytherins, but you've trained yourself not to wince.
Severus worries that maybe you do fancy Potter. You are astonished that he could possibly so malign your good taste, and you tell him so. He cheers up considerably.
Still, you have your doubts—not about Potter's appeal or lack thereof, but about Severus's newest friends. Frankly, Avery disgusts you. He's so twitchy—he doesn't even have the courage to be bad without shame, and you're surprised to find that this matters. Perhaps it shouldn't—evil is evil, as you've read somewhere lately—but somehow it does. Mulciber is simply a thug. You know this, and it doesn't take another round of "Evans, the Mudblood," to make you and everyone else with any sense see it.
Mary may be your least favorite roommate—you and Alice are getting quite close, and Marlene is lovably weird—but she doesn't deserve what they did to her. You don't understand why Sev doesn't see that.
Things between the two of you are changing, and you're not sure you like where this is going. He is the one person you trust the most in your life, he's taught you spells you never could have imagined, he's been your guide to a strange world, you can talk to him like you've never been able to talk to anyone, even Tuney, and you, at least, have definitely started feeling a spark.
But there really is something wrong with those Slytherin friends of his. If nothing else, you're pretty sure they actually refer to you as "Snape's little Mudblood," which, to say the least, is rather demeaning.
But, you tell yourself, you're a witch: you'll figure something out.