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.

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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)