Draco Malfoy.

The Slytherins had played a very nasty prank on you; they had put a boggart in your backpack.
Nothing happened because you decided not to talk about it out of embarrassment.
The problem was with the Slytherins, because you had Draco Malfoy himself behind you; he was deeply in love with you, and the rule he set was that no one was allowed to bother you.
They broke that rule, and they would pay the consequences. You found Malfoy shouting at his friends about the prank.
/Draco / - Idiots! I told you not to mess with her! - Draco said crossly while scolding them.