Severus Snape is an interesting character. There. Make of that what you will.
Note that I am not saying he is or isn’t sexy, evil, victimized, misunderstood, noble, or any other such absolute descriptive. He’s interesting. I believe JKR called him “deeply horrible,” (specifically in the context of discussing how young girls often develop crushes on boys who are mean to them, and how that isn’t a healthy behavior pattern). Snape is certainly a complete mess inside, with a completely twisted psyche. Frankly, I don’t think he wanted to survive the war.
But all of that helps to make him a very interesting literary character when one is reading a What-If story in which certain details are slightly changed, giving Snape chances to reveal a better, or worse, side of himself. I’ve seen fantastic stories in which he was the devil, and equally fantastic where he was this amazing, secretive hero, and others in which he was just a flawed man trying his best to do the right thing in spite of his bitter, spiteful nature.
In this fic, about the time of Sirius Black’s escape from Azkaban, Snape has an infuriating meeting with Dumbledore, who is doing nothing to prepare Harry for the war which the two men sense is coming. Dumbledore has decided not to even tell Harry the most basic of information about his supposedly murderous godfather. Snape feels that if Dumbledore actually took seriously Snape’s promise to keep Harry safe, that he would help Snape to do so, rather than demonstrating so much of a wait-and-see attitude toward the Boy Who Lived.
So, Snape steps in. Harry ends up getting training from Snape and gains a guardian at the same time (which makes this, loosely, a type of Sevitus fic). The events of Books 3 and 4 are changed dramatically because Harry has someone else helping him figure out what Voldemort is up to, and because Snape gradually has his eyes opened as to who Harry Potter really is. In both cases, that better understanding may be hinged upon a key point which both Snape and Harry (and Dumbledore and the entire flipping wizarding world, for that matter) have been missing: Harry is a child. Harry has needed an adult in his life who would help him think through problems and then assume the adult responsibilities while Harry got on with the business of being a student. And Snape has much less trouble with his past bigotry when he’s able to look at Harry and simply see a frightened child. Perspective really is everything.
There are layers upon layers of plot lines and deeper meanings interwoven all throughout this tale. Just when you think you’ve found all the connections, something else will startle you. And although I’ve focused this rec on Snape, the story’s alternating POVs are probably more focused upon Harry as the primary protagonist. What’s impressive is that the author kept track of everything in this carefully organized plot. “Far Beyond A Promise Kept” is concisely worded with snappy dialogue, fantastic, introspective characterizations, an adventurous, exciting plot, and a gifted author outlining and directing it all from start to finish.