Short answers, pulled from the story.
Heather Mason is a fictional character in Silent Hill 3 who was stitched together from the shattered remains of a burned child named Alessa Gillespie and a baby named Cheryl Mason. She is the physical manifestation of a soul that had been torn apart and serves as the protagonist of the game.
Heather Mason was created when the soul of Alessa Gillespie reincarnated into a new body seven years after Alessa was burned alive during a ritual by The Order. This fusion of identities resulted in a girl who carried the weight of a cult's failed godhood ritual within her own body.
Harry Mason was murdered by Claudia Wolf, a high-ranking member of The Order, to draw Heather back to Silent Hill. This murder served as the catalyst that set Heather on her path to confront the forces that had tried to use her as a tool for evil.
Heather Mason defeated the god by taking out a red pellet from a necklace given to her by her father and using the demon expelling liquid known as Aglaophotis to vomit up the fetus of God. This act ultimately led to the destruction of the god that the Order had tried to birth.
The character was initially modeled after French actresses Charlotte Gainsbourg and Vanessa Paradis before the development team changed the design to be inspired by Sophie Marceau and Gainsbourg. The character designer Shingo Yuri and the female team members of Team Silent decided on her final look including shorter curly hair and jeans.
Heather Mason appears in Silent Hill: Shattered Memories as the game's true protagonist and in the film Silent Hill: Revelation portrayed by Adelaide Clemens. She also appears as a playable character in the game Dead by Daylight and has been featured on various lists of the best video game characters.