And maybe Narcissa truly does love her, but Narcissa is ice. She is frozen, taught for so long to be cold and unforgiving.
Their time is brief. Stolen kisses in abandoned classrooms, whispered words under the safe cover of darkness. Narcissa goes through the motions, but she keeps her heart out of it, so afraid of that burning flame. She's cautious, calculating, and she plays it so very safe. The risk is too great, and she refuses to to let the ice in her heart give way.
Maybe she wishes she could show Marlene more than just the tip of the iceberg. There's so much beneath the surface, just out of human reach. But Narcissa is smart. If you stand too close to fire, you get burned.
When it ends, she doesn't cry, doesn't even blink an eye as Marlene issues her broken, passionate pleas. Without a word, Narcissa turns away, ensuring that she will not melt. Not for Marlene, not for anybody.