It's not in the way she speaks or carries herself or even the way she is forever offhandedly mentioning his faults to his father. A vapid, "Really, Severus could stand to bear himself a bit more like you, dear," when they deigned to take their thrice cursed responsibility out in public, or a giggly, "Dear lord, Severus," (such a Muggle, utterly Muggle curse) when he would say something she didn't understand. As if to suggest intelligence were something to be frowned upon.
His mother had been smart.
No, it was in her smile, the way the corners of her eyes crinkled up but the actual blue of her eyes was always a bit. . .glacial. Severus had never seen the icebergs of the Arctic but the books he had read on northern breeds of magic had contained pictures of that blue sheen. It reflected back on the sun beautifully and glossed over the magnificently quiet danger of the icebergs themselves.
She was always smiling.
Severus had never loved his father, not the dark man who he so very much resembled, the one who had driven his mother and the reassuring, familiar touch of her magic to such madness that in the end she could not remember Severus, let alone the stories she would tell him, the spells she would whisper with their long, exotic sounding words from a world that once was and would always be.
His stepmother came after, once his father had safely packed away his quiet, unassuming, witch (the word always sounded like a slap in Severus's head) of a wife. The new wife had appeared then. She of the iceberg blue eyes and ready smile and wit that was sharp in ways that didn't cause people to yell at her.
People yelled at Severus.
She who didn't smile the day they got the letter. Instead she said, "Are you sure of that, Tobias?" to his father's emphatic exclamation that Severus would never attend his mother's alma mater. "After all," she purred, still not smiling, "the term is nearly year round."
When Severus hauled his things up onto the Hogwarts Express all by himself, he found an empty compartment, and closed the door to hide his smile where nobody would see it.