She looks intimidating
She is a very tall, slender woman with long black hair that extends past her waist with locks of hair that frame her face down to her chin, pale skin, large breasts, a high forehead, and dark brown (dark blue in the pre-time skip anime then switched to dark brown after the time-skip to match the manga) eyes that have long, voluminous lashes.
Like most of the females in the series, she has a narrow waist.
During her escape from Mariejois, she wore rags and tied her hair in pigtails.
She then wore a qipao with a hooded cloak which she used it to cover Luffy until they arrived at Impel Down and then took it off during the battle against Whitebeard.
Along with her sisters, on her back is the slave mark of the Celestial Dragon, something that she was so intent on keeping secret and was willing to petrify anyone (other than her sisters and Elder Nyon) who saw it.
In order to make sure it was never seen, she and her sisters concocted a lie that what was really on the sisters' backs were pairs of Gorgon eyes which would petrify anyone who sets eyes on them.
Hancock uses this "curse" as an excuse for her and her sisters' Devil Fruit abilities.
As a young child, she looked similar to how she does now, only her side-locks went past her shoulders and were worn in beads.