Well , I guess if you are talking about personality, she was probably known as the angel of the family , the flirtatious , a sweet and kind-hearted girl , that is always there ,ready to help those in need[like she used to do with Alexandra].
Now , if you are referring to her physiognomy , she was known , of course , for her blue eyes.The Empress worried about her weight when she was younger , wich means she might have been plump in her childhood , though as she grown older , the fatness started to desappear...
I also read that people used to say she had a typical Romanov eyebrow.
[OH , BY THE WAY, ENGLISH IS NOT MY NATIVE LANGUAGE , SO IF THERE ARE ANYTHING WRONG ... I'M SORRY! =/ ]