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.

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.
(Source: One Piece Wiki)