The first time Draco takes notice of Neville Longbottom - really takes notice, not singles him out as an easy target to pick on - the boy is standing in front of him, trembling in a wash of fear, guilt and excitement, shoving his fists up and calling "I'm worth ten of you, Malfoy!"
He doesn't do anything but deride him, right then. Can do nothing else. But the image - the near-squib Draco had dismissed, all riled up and getting over his fear like it was nothing - stayed with him.
Five Years later and Longbottom is in the crowd that defies Umbridge's orders, in Potter's inner circle that puts Draco's father in prison. Anyone paying attention to anyone other than Potter at all would know there is no ignoring the boy – the man.
He and Finnigan, Lovegood and the Weasley girl are the ones holding the student body together during seventh year. As far as Draco is concerned, with the others so anxious about the members of every house but Slytherin, Longbottom might as well be the only one.
There is no real communication between them. They have never, in all the years they've known each other, had what could pass for a conversation. But Longbottom's gaze is observant beyond being monitoring, and it does mean he sees the way some Slytherins are struggling under the Carrows' rule, sees the way there are things Snape cannot control, and sees the little things, the very few little things that are possible for Draco and his peers to do.
They haven't the capacity to protect anyone but their own people. Not that they would, necessarily, but they have no maneuvering space whatsoever and thus never have the opportunity.
Finnigan and Weasley wouldn't see it that way. The teachers wouldn't, McGonnagal least of all. Lovegood might. That Longbottom does and lets Draco know he does without ever saying a word is the reason he eventually becomes Neville.
Neville fights hard for everyone. Draco fights just as hard, with little to no visible results. Neville leads the underground, lets Potter-Granger-Weasley into the castle, moves the earth, kills a snake. Draco loses what until that very moment never felt like a close (best) friend and is nearly thrown into Azkaban.
He isn't, because when Draco cannot fight anymore and there is nothing left that his own mother can do for her son, Neville fights for him.
That, combined with the fact that Narcissa saved Potter's life, and Potter saved Draco's life and presumably feels responsible for him, is enough to save both him and Gregory. In fact, when the dust has settled, Draco finds that it saved (nearly) every Slytherin student, even Pansy Parkinson.
Slytherin students fought against the Death Eater oppression. Neville has evidence.
Draco has no doubt the Hogwarts Resistance Quartet has evidence for the crimes Slytherin students have committed, as well, but that is not what they submit during the trials. Not Longbottom. Not Finnigan. Not Weasley. Not even McGonnagal.
There is a debt, there, that will need to be paid but the price will not be set by anyone who did not see, by judiciaries whose positions were void, who had nothing to do but hide, who weren't there.
The man Draco once regarded as a squib is saving them all, and Draco- ...Draco -
Draco can do nothing but continue to fall.