From Sonic Retro
Maria Robotnik was the granddaughter of Professor Gerald Robotnik, and consequently a cousin of Dr. Eggman. Living on the Space Colony ARK with her grandfather during his work on Project Shadow, Maria became friends with the Professor's creation, Shadow the Hedgehog (as well as the young GUN Commander). She was killed during the GUN assault on the ARK 50 years before Sonic's time; in her last seconds she saved Shadow from GUN troopers by jettisoning him from the space station in an escape pod. Shadow made a promise to her to bring hope to humanity and give humans a chance to be happy. Despite some mental sidetracking, Shadow fulfilled that promise not once but twice, defeating the global threats of Finalhazard and later Devil Doom.
Maria suffered from an illness known as "NIDS" (Neuro-Immune Deficiency Syndrome), and was considered incurable at the time. Prof. Gerald's research aboard the Space Colony ARK was also intended to try and find a cure to this illness, of which, "The Search for Immortality" was hinted at by the President. Gerald reluctantly took on the task to try and save his granddaughter, and named it Project Shadow.
Trials of the test had been practiced on the Biolizard, which quickly became unstable with the power of the Chaos Emeralds and required a Life Support System to continue living, and before Project Shadow's final tests before being applied to humans, the ARK was stormed by G.U.N., who then closed down the research facility and locked away the only chance for mankind. Shadow and Maria evacuated to Professor Gerald's laboratory, only to find Gerald injured and a giant GUN battleship inside called Heavy Dog. Some nearby GUN soldiers attempted to shoot Shadow, but Maria took the bullet and died. Shadow tried to avenge her by destroying Heavy Dog, with the help of the evil Black Doom.
Maria is a very kind girl, and is about twelve years old. In Sonic Adventure 2, she wants Shadow to protect mankind. In Shadow the Hedgehog, she also vows to protect Earth, and feels sorry for dead GUN soldiers and injured people, and does whatever she can to help them. Maria is concerned about Earth and humans' safety, especially Professor Gerald's.
Shadow the Hedgehog
She returns in Shadow's new game as a mission character. In the Action Stage The Doom, Maria wishes for Shadow to heal ten of the wounded researchers, not caring about the fact that 61 GUN troops are after her. And in the Action Stage Lost Impact, Maria and Shadow go on a mission to shut down the Artificial Chaos before they destroy ARK.
Maria was actually playable in these two missions; as in every Shadow level, the secondary character could be controlled by a second player. Unsurprisingly, though, Maria could not perform spin attacks and so was essentially useless in gameplay.