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.
(Source: One Piece Wiki)
