Mary Hatch Bailey

Mary Hatch Bailey is a fictional character in Frank Capra's 1946 film "It's a Wonderful Life". She is the high school sweetheart and later wife of protagonist George Bailey (played by James Stewart as an adult and Bobby Anderson as a child). Mary is played by Donna Reed as an adult and Jean Gale as a child. She is loosely based on Mary Pratt, a character in Philip Van Doren Stern's 1943 short story "The Greatest Gift". 
Mary Hatch is viewed by some as the hero of the film.
Story line.
Mary Hatch is the younger sister of Marty Hatch, a friend of George, whom she has loved since childhood. They become reacquainted as young adults at a high school dance in 1928, where they discuss their respective plans for the future. They are interrupted by news that George's father has suffered a stroke. Mary has been away at college, and worked for a time in New York before returning to Bedford Falls.
In 1932, when George's brother Harry returns from college with a new wife, and a job offer from his father-in-law, George realizes he will not be able to leave the Building and Loan, at least in the foreseeable future. George, despite knowing that his friend, Sam Wainwright, is courting Mary, goes to her house at the urging of his mother. George insists he that he has no intention of getting married, but realizes that he loves Mary. It is very clear that George's mother is right that Mary loves him and not Sam.
George and Mary are married, but their planned honeymoon is interrupted by a run on the banks, which affects the Building and Loan. Potter's newly acquired bank has called in its loan, and depositors are clamoring to withdraw their funds. Potter, a stockholder in the Building and Loan, threatens to shut it down if it closes early. Mary offers the money saved for their honeymoon to keep the Building and Loan solvent.
That evening, George's friends Bert the cop and the cab driver Ernie escort him to his "new" home: the Old Granville house, a derelict mansion Mary had wished for years before. Over time they work on repairing the house. During World War II George is exempt from the draft because of partial deafness, but serves as an Air-Raid Warden. Mary becomes a volunteer for the war effort, such as running the local branch of the USO, while raising their four children (Pete, Janie, Tommy, and Zuzu). In 1945, when George's uncle misplaces an $8,000 deposit, George faces bankruptcy and a charge of embezzlement. Angry and depressed, George contemplates suicide. His guardian angel, Clarence Odbody, then shows George what life would have been like had he never been born. Among other changes in this alternate existence, Mary is a spinster librarian.
George returns home to find that Mary has rallied their friends and family to raise the missing money.
Behind the scenes.
Donna Reed would later state that "It's a Wonderful Life" was her favorite film of her career.
In the "wishing" scene outside the old Granville house, Capra was surprised to see Reed accurately throw a stone to break a window, a skill she acquired playing baseball with her brothers on their Iowa farm.