Names
Many knew her in her late teens and early adult years as Ma Chao the Splendid's Little Sister. Then people started calling her by her real name, Ma Yunlu, and from there, some would address her as Cixi.
But to him, she was always "Lulu". It was embarrassing if he accidently slipped in public (a rare occurrence, but horrifying nonetheless), but when they were alone she thought it was the sweetest thing anyone had ever said to her. No one else called her by that name, and no one ever would. It was his name and his alone.