Hello! Here for your requested review!
This was truly a beautiful one-shot and my heart broke while reading it. I was craving for some hardcore angst and this was exactly what I needed xD I really enjoyed reading this (not sure if 'enjoyed' is the right word though) and I think you did an amazing job writing this =)
To address your concerns, I really think your narrator's characterisation was very well crafted. I am so intrigued by who he could be. Severus is the first thought that comes to my mind, since he grew up with Lily as kids, and then he wanted her, but lost her, and ultimately she died and he wasn't able to tell her he loved her, but of course it could be anyone. Nevertheless, you wrote him beautifully, expressing his pain, his nostalgia, his depression, very accurately. I could really sympathise with him.
The woman's character (or should I say Lily's) was well-written too. Even though we saw her through the narrator's eyes, I felt like I really knew her. I especially loved your descriptions about her beauty and such.
The story flowed very smoothly too. The transitions from one "scenario" to another were quite neat. The pace of your writing was fine too, it was slow enough to go with the melancholic theme yet not too slow to get boring. I liked that you kept it short and didn't make this very long, as then it would have become repetitive. I did feel slight repetition in the story, but it since it wasnt dragged further, it was not intolerable.
As for realistic-ness, I think this was very believable, especially if I put Severus/Lily in the place of main characters here. I would have liked to see more detail on the reasoning as to why he gave her up though, more concrete on the reasoning would have made it a little more realistic too. But apart from that, it was pretty okay.
The plot and the concept of course was well-thought out and executed brilliantly. I liked the whole concept of keeping the names hidden as it maintained a great air of intrigue.
So, all in all, I don't have much CC to give you, except for the little thing about there being slight repetition of sorts in the piece, and the reasoning behind him giving her up having a little more detail.
Besides that, Great work, over all. I really loved reading this, especially since the writing style was so good, and the grammar was fine!