It's hard to get a handle on how to praise this fic because it's so all of a piece, and it's the flavor of Filch's voice that makes it so. You have a knack for casting his remarks and mullings-over in marvelous turns of phrase, compulsively interesting, partly because they come from such an outsider perspective. All of his observations are dry, wry, unvarnished, stinting, oddly scrupulous even when they're reprehensible, self-deprecating but gimlet-eyed, morally rigid and shrewd. Not to mention funny. He's excellent company, in his own misanthropic way.
His opinions of Fudge and of Umbridge's flutterings are spot-on. Then, the thought of Umbridge fancying Snape makes me shudder as much as it does Argus. Along the same lines, I love how the simple use of Severus' name makes it clear that Argus sets him apart, that his attitude toward him is personal, not professionally dismissive or deferential. I also like the sympathy you stir with Argus' reminiscence about the manacles and how he thinks of them as his 'talisman.' His obsession with them is suddenly understandable, given how they touch his magical heritage, which is also the source of his deprivation.
Actually the absorbing portrait of Argus is what lingers on after the fic ends. I love the portrayal of Severus and Minerva glimpsed through Argus' limited viewpoint. How enamored he is of Severus' body. How even his vitriolic descriptions of Minerva bring her crackling to life and only make me fangirl her more.
But it's Argus and his backstory that cast a spell, his memories of being sent to live with Muggle relatives who treated him as harshly as the Dursleys treated Harry, the sense of not being 'right,' of being looked down upon, of holding to an unspoken pride in how much he knows that the wizards around him don't even suspect, his fierce belief in how the world ought to be. It all fits: Argus as loner, an outcast on the fringes of a fabulous, special place into which he was born but to which he's denied true entry and full citizenship.
Which then makes perfect sense of his desire to watch and not be touched. He's always the one looking on; he's always lurking, the caretaker of many secrets, aware of hijinks and rendezvous unknown to his 'betters.' It makes his name a prescient one. And it adds to that sense of loneliness and punishing honor that, at the last, makes his prior loyalty to Severus, and even to McGonagall, mean more to him than being prized by the likes of Dolores Umbridge. (Well, loyalty and a healthy respect for his own skin.)
All right, I can't help it, I have to quote your story to you, thereby no doubt breaking the LJ character limit:
not with all them cats on the wall -- he preferred his cats alive, thank you kindly How can I not like this man? I want to follow him around, eavesdropping on his running interior monologue.
not wanting to hear his mam agree that he weren't right and that sending him away were. / No, it hadn't been bad. But Argus still didn't know if it had been right.
This is touching, even if he has found a way to live with it. But yes, it's the sort of thing that would knobble up someone's sense of self, like premature arthritis.
"Not everybody can make sense of word talk. But there inn't nobody don't understand pain talk." This is brilliant as the kernel of Argus' philosophy of life. Of course it's how he's been brought up. As he suffered and learned, so should others. I like the crustiness and the unreflective cruelty.
It were folk like him what most needed a world they could count on Sharp bit of character insight.
And now that he'd got the hang of ignoring them kittens on the wall Hee!
She had about as much chance of getting Severus into her drawers as Mrs Norris had of becoming Minister of Magic. This is witty, and delightfully matter-of-fact.
Severus were a man who made people feel things -- strong things, dark things, needy things. The Headmistress weren't the first person to want him. To want to understand him or save him or join in his righteous darkness. I love what this reveals about both Severus and Argus. I especially like the reference to 'righteous darkness' and how you leave the meaning of that ambiguous.