Mena
Little is known about Mena's past, except that she and Bambi's mother were childhood friends who grew up together.
After Bambi's mother was shot and killed by hunters, the Great Prince asked Friend Owl to find a suitable doe to raise Bambi, because he was taught that it's the doe's job to raise fawns. When Spring came, Friend Owl found Mena, who was happy to become an adoptive mother to Bambi. At first, Bambi is shocked to learn that his father was sending him away, and he yells at him that he wishes his mother was there instead of him, and he runs away to tell his friends. The Great Prince tells Friend Owl: "This is my fault. Bambi needs a mother. A prince should NOT be raising a child!"
Bambi eventually goes with Mena to her home, but Ronno appears and taunts Bambi, saying that his father sent him away, because he was ashamed of him. Bambi fights Ronno, and Mena gets caught in one of Man's snare traps. The ringing of the bells attached to the trap beckons Man's hunting dogs, and Ronno screams: "Mommy!" and runs away.
Mena repeats Bambi's mother's last words before her death, and Bambi is determined not to let history repeat itself. Bambi lures the hunting dogs away, and the Great Prince frees Mena from the snare trap.
When Bambi regains consciousness after he's nearly killed after falling off the cliff, he and his father reconcile, and Mena decides that she's no longer needed, and leaves.
TL;DR
Mena was Bambi's mom's friend. After Bambi's mom died, Bambi's dad asked Owl to find a new mom. Mena agreed. Bambi was mad at first, but they made up. Mena got caught in a trap, Bambi saved her, and then she left.