Mal Cobb was the wife of Dom Cobb. She met him at university, through her father, a professor of architecture. She had two children with Dom, Phillipa and James, and together with her husband explored the art of shared dreaming.
When experimenting with dreaming Dom and Mal went too deep and found themselves in Limbo. They built themselves a city, and while Dom rejected the reality Mal lost sight of her dreams and believed she was in the real world. Dom performed Inception on her to plant the idea in her mind that her world wasn't real. They killed themselves in the dream by resting their heads on a train line.
However, after waking Mal still believed she was dreaming, and refused to accept the real world. Eventually she told their lawyer that she feared for her safety, then smashed up her hotel room and jumped out of the window, killing herself. Dom didn't follow suit, and so he was implicated in her murder and had to leave America and their children. Mal continued to haunt her husband's dreams after her death.