Boa Marigold is one of the three Gorgon sisters that rule over Amazon Lily. She and her sisters are called the Gorgon sisters for supposedly heroically slaying a monster called the Gorgon. She is the younger sister of Boa Hancock and Boa Sandersonia.

Appearance; Personality; Abilities and Powers; Devil Fruit; Trivia:
Appearance:
When she was younger, Marigold was quite small and slim like her older sister. Now she is a large and fat woman. Her hair is arranged in a way that it looks like the patterns on a cobra's hood and to some extent, a Valkyrie's helmet. She is first seen accompanied with a large hawk at her side instead of a snake like the other Kuja. On her back, covered by her cape and her long hair, is believed by the Kuja, a set of eyes she and her sisters received as part of a curse for slaying the Gorgon. It is now known that the symbol on the Boa sisters' backs is the "Hoof of the Soaring Dragon," an emblem burned upon the slaves of the Tenryuubito.
Personality:
Marigold is the stern one amongst the three sisters, who displays the least amount of emotions outside of battle. While she appears to be living on the lie that her sisters and herself defeated a Gorgon to gain their curse, at the same time she hasn't actually told a lie herself unlike Hancock who is constantly lying.
In battle Marigold is an offensive warrior who is slightly playful (but not as playful as Sandersonia).
Abilities and Powers:
Ruling beside her sisters, Marigold has power over Amazon Lily and the rest of the Kuja women. Being apart of her sister's pirate crew, she is one of the elite Kuja warriors.
Also of the sisters, she is the only one that actually wield a weapon, in this case a spear.
She possesses the ability to use Haki to defend and deflect against attacks, even against other Devil Fruit users. While she can do this, she can't block an opponent's attack if it is too strong for her, like Luffy's Gear Second attacks.
Devil Fruit:
Marigold ate the Hebi Hebi no Mi, Model: King Cobra that allows her to turn into a King Cobra. While these powers are Zoan Devil Fruit based, the Kuja of the island attribute Marigold's and Sandersonia's ability to turn into snakes as part of the curse.
Trivia:
Her name comes from the flower Marigold.
While Marigold and her sisters' title, the Gorgon Sisters, comes from supposedly slaying a monster with the same name, their concept is loosely based on the three actual mythological Gorgon sisters which includes the famous Medusa.

(Source: One Piece Wikia)