Bellemere

Bellemere was Nami and Nojiko's adoptive mother, seen in a flashback during the Arlong arc.

A former Marine, Bellemere came across Nojiko and Nami in the ruins of a town in the aftermath of a great battle. She decides to leave the Marines and raise them as her daughters, in order to get away from the death and destruction of Marine life.

[Spoiler] When Arlong arrives at her island, Bellemere is the only person to fight back with any degree of success. However, she's caught off-guard by the fishmen's strength, and allows Arlong to kill her in exchange for Nojiko and Nami's safety. [/Spoiler]

(Source: One Piece Wiki)