This is primarily a Neville Snape mentor story, but I expect Harry will work his way in, the incorrigible child. I'd love to hear what y'all think...
He hears the whispers, of course. The same snickering laughter that had always seemed to follow in the Marauders' wake. There is no sympathy in his colleagues' eyes, merely stifled amusement.
It's only what you deserve.
This once, Severus Snape cannot fault their judgment. He has a role to play, and he has played it to perfection. What better way to cement his image as cruel, heartless, and Dark Arts obsessed Potions Master than to ceaselessly and remorselessly bully poor, bumbling Neville Longbottom?
But he'd never imagined he could be anyone's worst fear.
Hated? Pitied? Ostracized and despised by all except those he so blatantly favors? Yes, of course—this is his goal. But he'd never thought he could instill so much terror that he is a child's Boggart.
He is ashamed, and he is tempted. Imagine their faces, if but for one day he could abandon this careful balancing act. If for one day he could allocate points and punishments as are deserved, without fear of repercussions. If for one day he could simply teach, and govern the most dangerous class in all Hogwarts through reasonable, common sense safety measures, rather than sheer fear and a spy's attention to detail. He's lost count of how many thrown potions ingredients he's silently Switched with something more harmless, and just once he'd like to verbally lambast Draco Malfoy for his malice-ridden stupidity.
It is a fond dream he will never see, because there would be hell to pay, and he's not so foolish as to think he'll survive the coming war.
But today, as laughter dogs his footsteps and judgement shines in every gaze, Severus Snape decides that something, something must give. He is a liar and a spy, who betrayed the one person he ever loved. He is bound to a path he dare not falter upon, lest the last thing he holds sacred fall to ruin.
But he is damned if he will be Neville Longbottom's Boggart any longer.
He scours every inch of his old textbook. This is not his copy of Advanced Potion Making, and it holds no dark spells or defiant appellations. But this is the book he used his first five years of Hogwarts, those golden years with Lily, and its secrets are far less dark and far more damaging.
Her name is not present anywhere, but he traces her handwriting where it appears in their scribbled arguments, and for a moment he falters, unsure if he can bear to part with it, after all. In the end he swallows, and obliterates only his name on the inside front cover. Perhaps, even in their childish arguments over additions and modifications, Lily and Sev can teach Longbottom what Professor Snape cannot.
He dashes off a note, allowing his handwriting to slip into the same cramped scrawl that fills the textbook's pages. A student in handwriting could trace his elegant professor's screed to its humble origins, but only if they are looking for a likeness. Severus does not intend that Longbottom should ever have reason to look for one.
Dear Mr. Longbottom,
I have heard through my connections to the school that you have a most difficult relationship with the current Potions Master. As a master in the Art myself, it appalls me that any student should be forced to study it under a professor he fears.
The staffing decisions of the Headmaster are not within my power to affect, but I offer you two things which I hope might alleviate some of your distress.
First, a word of advice: your Professor Snape has only what power you give him. He cannot, no matter what he threatens, do anything more than assign you detentions and take points. He is infamous even among his peers for his unpleasant character, unsavory history, and the vitriol of his tongue, and should he leave Hogwarts and the Headmaster's protection he would find it difficult to establish himself elsewhere. Thus he might threaten and bluster, but he cannot truly harm you without destroying his prospects. I do not advise you to confront him, but to allow his taunts and vitriol to slide off you, secure in the knowledge that you are safe, and that Severus Snape is merely a bitter and small-minded man with no future.
Second, a more tangible aid, which you will no doubt find easier to implement. The package accompanying this letter is my own potions textbook from when I was your age, complete with modifications and discussions of theory. While merely using my adaptations will improve your performance, I strongly urge you to think about the reasons for those changes. Much of the theory of potions lies in an understanding of the properties of ingredients and their preparation, and the greater your understanding there, the easier the brewing will be. If you truly wish to understand potions, and not merely scrape through, I recommend a thorough study of 1001 Magical Herbs and Fungi, A Primer on Proper Ingredient Preparation, and The Basic Theorie of Potiones, all of which should be available in the Hogwarts Library.
I wish you the best of luck in your endeavors, and humbly request that you keep my interference hidden, even from your friends.
Longbottom's face as he reads the letter is a study. Severus is grateful that no pays much attention to the boy, lest someone realize that something is up.
Wariness gives way to confusion, incredulity, and something that might just be hope, before settling into an expression of thoughtfulness that seems utterly foreign on Longbottom's face. He feels a small, unfamiliar glow of satisfaction inside him, and so is not prepared for the way the boy's gaze flies up and fixes on him. He sneers, almost automatically, and Longbottom blanches, the familiar terror returned.
Severus Snape is not prepared for how much that terror hurts him, like a blow that steals his breath and replaces it with bitter disappointment. He swallows, feeling his face retreat into its customary mask, and focuses on taking deep breaths. Of course. Of course, a few words from a stranger would not be enough to overcome two years of fear. Hopefully the boy will take advantage of the book, giving him an excuse to be just a little less abrasive in class. If not, he'd have to do something else.
This would bear thinking upon.