Short answers, pulled from the story.
Heather Mason is the main protagonist of Silent Hill 3, released in 2003 by Konami. She is the reincarnation of Alessa Gillespie and Cheryl Mason, and the adopted daughter of Harry Mason. She first appeared as a supporting character in the original Silent Hill in 1999.
The Aglaophotis is a liquid capable of expelling demons from the body. In Silent Hill 3, Heather Mason uses it to vomit up a divine fetus that had been growing inside her. The substance was contained in a red pellet hidden inside a necklace her father gave her.
Heather Mason was voiced and motion-captured by Heather Morris. The character's name and visual design were both inspired by Morris, who drew on childhood taekwondo training for the game's combat sequences. The creators originally named the character Helen before changing it after working with Morris.
Silent Hill 3 has three endings. In the normal ending, Heather meets Douglas and decides to start going by Cheryl. In the possessed ending, she kills Douglas instead. In the joke Revenge ending, Harry Mason sends an army of UFOs to destroy Silent Hill. Silent Hill: Homecoming later indicated the normal ending is canon.
Heather Mason appeared in Dead by Daylight in 2020 as a playable character going by the name Cheryl. She was added alongside Pyramid Head, Lisa Garland, Cybil Bennett, Maria, and James Sunderland. In Dead by Daylight's own canon, Cheryl works at a center for troubled youths but remains haunted by Silent Hill 3.
Adelaide Clemens played the reborn Heather Mason in the 2012 film Silent Hill: Revelation, where Heather is the main character. The 2006 Silent Hill film did not feature Heather directly, instead depicting Alessa Gillespie, played by Lorry Ayers as an adult and Jodelle Ferland as a child.