Oh, this is so absolutely beautiful. I love everything about it - the love for Lily and the comfort with Filius, the gradually changing attitudes of the Hogwarts staff, your wonderful, wonderful characterisation of both of them. I particularly love the contrast between them - Filius with his pastels and cheery smiles and Severus with his quiet simplicity, and the way they complemented each other. I felt Severus' growing sense of belonging acutely - you captured it really well, along with the feeling of everything beginning to unravel again. The goodbye, if I can call it that, was heart wrenching - and just like Severus to be cruel in order to be kind. And the ending, oh, the ending! Sweet and perfect and not too sentimental and just gah.
This was magnificent. Thank you for writing such a wonderful fic.