Marie Heinlein is the older sister of Kain R. Heinlein. She lived happily with her brother and their friend Abel Cameron. Eventually, they grew up and Marie attracted the attention of the crime boss, Geese Howard. They were married, but Marie had to live always hidden from the world, because she would represent a weakness to Geese's enemies. One day, she discovered she was pregnant. She had a boy and named him Rock. So, she lived her unfortunately sad life with her son, until one day, she fell ill and passed away. Years later, Kain invited Rock to a King of Fighters tournament in order to tell him the truth about his mother. By the end of such tournament, Kain mentioned that Rock's mother is alive. In the fan Q & A for the game, creators noted that the confirmation of her death wasn't made by a physician, thus leaving some room for error about her fate.
Although the sequel has not been available to the public, many fans speculate over the real reasons behind her apparent death. There are others who even question Kain's relation to Marie. Though creators have expressed interests in their opinions, they have not stated anything about either ideas.