This is the character in 2002 movie, for 2018 movie, see Maria (Pirate).
Maria was a nursemaid robot who worked under King Atomu along with her son, Poko. At the time the queen had passed away so the king entrusted her to look after the princess, Jeanne. Maria loves Jeanne like her own daughter and the princess has come to see her as her mother and Poko as her little brother as well.
However, the relationship between them was strained when in Jeanne's teenage years, King Atomu was killed in an accident when he tries to save one of the worker robots. With the Royal Advisor Dester's manipulation of the princess's sadness and despair, Jeanne believed that her father died because the robots have emotions. After she became the queen, she decreed that all the robots must have their emotions removed. Hearing about this, Maria and Poko fled the palace and ran away with Jeanne sent the guard robots after them. While Poko managed to escape with the help of Doctor Chapek, Maria was captured and imprisoned. Despite Jeanne telling her it's just a "nursemaid robot", she didn't order to have Maria's emotions removed. This is suspected by Dester that it is because Jeanne still thinks of her as a mother figure.
When Doraemon was captured, he was imprisoned in the same room as Maria as Dester was unable to remove his emotion. Doraemon was able to recognize her as Poko's mother thanks to their similar earphones. Worrying, Maria began asking Doraemon about Poko's whereabouts which he replied that Poko is safe now which made her relieved. Maria then shared her own story about her and Poko's relationship to Jeanne and Dester's manipulation. However, Maria told Doraemon that she believe that Jeanne herself may know that she changed and she still believes in her.
Later when the gang along with Poko, the reformed Jeanne, Dr.Chapek and people from the Rainbow Valley began their assault toward Dester, he ordered one of the robot soldiers to bring Maria to him at the control room. There, Dester told her that Jeanne, Poko and everyone are going to die and transformed the castle into a fighting mecha and begin wreaking havoc, causing the group to flee. Luckily, with Nobita's quick thinking, he was able to foil Dester's plan by destroyed the legs of the mecha. Doraemon then rushed into the castle to save Maria. However, Dester managed to transform the control room into a rocket that quickly flies into space, homing toward the moon and when Doraemon knocked him out, he activated the self-destruct sequence on the rocket where it will explode between earth and moon.
When the hope seems lost and both Doraemon and Maria believed that they will perish, Poko's robotic sense turned out still able to sense her location. With aid of Dr. Chapek's machines, Poko managed to open the portal to their location, allowed Maria and Doraemon returned to Poko and everyone. This, however, also allowing Dester to return with them, only for the tyrant incapacitated and his true identity exposed. But nevertheless, Maria eventually reunites with Poko and Jeanne after separated from them for so long.