What a great story! I enjoyed every second of reading this - which is surprising really, given that I'm not fond of chan, cross-gen, or student/teacher and find guardian/ward a little incest-y (am quite conservative, as you can see.)
Having said all these dreadful things, I believe it's very much a credit to your story-telling skills that I loved this so. A good story that places these conditions in a convincing and engaging contexr seems to dispell all qualms!
Following Severus' perspective was fascinating, from the time of his despair, through his developing affection and his later confusion, culminating with your satisfying conclusion. I especially enjoyed the little details, like Severus' sulk when he thought Harry no longer had time for him, and his surpriseat being Harry's treasure in the Tri-Wizard tournament. Lovely work.