Joyce Summers's History
Early life
Joyce was born in 1958 and had one sister, Arlene, who would later live in Illinois along with her own family. Her niece, Celia, whom Buffy was very close to, died at a young age.
In high school, Joyce had a Gidget haircut and was on the yearbook staff. Joyce met Hank Summers at the prom during her freshman year of college. Joyce did not have a date, but Hank did. The two married and had one daughter, Buffy Summers, who they raised in Los Angeles.
By 1996, Buffy was called as the Slayer and, despite her reservations, accepted her role as such. Her secret identity clashed with Buffy's home life as Hank and Joyce's relationship eventually deteriorated. Their arguments included those about Buffy when she came home after her first slay.
Hank and Joyce placed Buffy in an asylum after she was expelled for burning down the Hemery High School gym in her battle against the vampire Lothos. Neither Hank nor Joyce believed that Buffy was a vampire Slayer and, to appease them both and leave the asylum, Buffy hid her identity from both.
Sunnydale
Mother to the Slayer
Joyce seeing Buffy off to school on her first day at Sunnydale High School.
After Hank and Joyce's divorce, Joyce gained custody of Buffy and made the choice to move to Sunnydale, where she managed an art gallery. Buffy made efforts to please her mother, while simultaneously balancing her secret life as the Slayer, often which Joyce interpreted as teenage rebellion. While protecting and always ready to offer guidance, Joyce could also be strict with Buffy, situationally grounding her and being disappointed in academic failure. She also became a maternal presence in the lives of Buffy's friends, Willow and Xander.
For Buffy's prom, Joyce bought Buffy with a white prom dress she noticed Buffy had her eye on, and recounted the story of how she met her father, unknowingly encouraging her to fight fate and the Master.
When vampires attacked Sunnydale High School on Parent-Teacher Night, Joyce protected her daughter by attacking Spike with an axe after others fled the school. She was concerned about Buffy's poor academic performance, but she was proud to discover that Buffy was a strong leader who took care of herself.
Joyce was disturbed when Angel revealed to be a stalker after losing his soul. She was even more upset when she learned that Buffy had lost her virginity to him. Joyce became increasingly frustrated with the distance between herself and her daughter. When police officers visited the house and informed her that Buffy was a murder suspect, Joyce feared the worst. She met Buffy, who was accompanied by Spike, outside their house. Buffy initially tried to convince her mom that she and Spike were in a band, but when the three were attacked by a vampire, Buffy was forced to stake him and once again told Joyce she was a vampire slayer.
After making a deal with Spike, Buffy then explained the situation to Joyce, who attempted to rationalize the situation and simply call the police on Angel. Frustrated and running out of time, Buffy forced Joyce to understand it was her duty to stop Angel. Joyce reacted badly to Buffy's callousness of the situation, demanding more time for her to explain. In a heated argument, Joyce warned Buffy to not return if she left. Grieving from the loss of Angel, Buffy took her word and departed Sunnydale for Los Angeles, leaving behind a letter for her.
While Buffy was gone, Joyce worked with Rupert Giles and the Scooby Gang to locate her. She developed a close friendship with Pat, a woman from her book club who was later killed by zombies. Joyce was relieved when Buffy returned and wanted things to go back to normal. She tried to support Buffy's extracurricular activities while also encouraging her to think about her future. After accompanying Buffy on patrol, Joyce fell under a demon's spell, which drove her to lead Sunnydale on a witch hunt. She and others attempted to burn Buffy, Willow, and Amy Madison at the stake and was later horrified at her actions.
Spike stopped by the Summers' home when he returned to Sunnydale where Joyce lent a sympathetic ear as he related the details of his recent break-up.
For Buffy's Tento di Cruciamentum, a rite of passage practiced by the Watchers Council to test the Slayer, they pitted her against Zachary Kralik, a psychotic vampire who, in life, had been a serial killer who tortured and murdered young women. However, he escaped captivity and abducted Joyce, taking her to Sunnydale Arms so Buffy would come to him to fight. With this, her Watcher Giles finally revealed to Buffy about the test with the intention of canceling it, and Buffy, even powerless, was able to save her mother and defeat the vampire.
Mother to the Key
In 2000, the ancient Order of Dagon decided that Buffy as the Slayer was the only warrior strong enough to protect a mystical energy known as the Key. In a ritual, they manage to grant the Key a new form as Joyce's second daughter, Dawn Summers. History and memory was rewritten, as to maintain the illusion that Dawn had always existed in Joyce' life.
Later on, Joyce's health deteriorated and she was diagnosed with a brain tumor. Pressure from the tumor interfered with her mind and she temporarily lost mental clarity. At that time, Joyce recognized Dawn's anomalous nature and confirmed with Buffy that Dawn was not her daughter by birth. Despite the revelation, she loved and accepted Dawn as a member of the family and urged Buffy to do everything in her power to protect Dawn.
Joyce's health improved after successful surgery, although her life was soon in danger again. Glorificus, a hell-goddess, sought Dawn in order to return to her home hell dimension. Although Glory did not realize that the Key was, in fact, Dawn in another form, she knew that Buffy protected it. She threatened Buffy's family, which forced Joyce and Dawn to temporarily stay with Spike for protection. She continued her friendly relationship with Spike and they watched the soap opera Passions together.
Joyce met a publisher named Brian who took her out for dinner and a movie. They had one date that went well, and he left her flowers the next day. Soon, Joyce suddenly died from a brain aneurysm.
Legacy
Dawn, devastated by Joyce's death and feeling abandoned by Buffy, who was picking up greater responsibilities and trying to avoid emotional breakdown, resorted to necromancy, despite Willow and Tara Maclay's warnings. With help from Spike and Doc, she performed a resurrection spell for Joyce. However, Dawn realized the inherent dangers of what she has done and that her mother may return as a zombie and broke the spell before Buffy could open the door and allow her in.
After her own resurrection, Buffy was poisoned by a Glarghk Guhl Kashmas'nik demon, which caused her to have visions of a version of herself incarcerated in a Los Angeles mental institution, and nearly became convinced she wasn't a Slayer. Joyce appeared, still married to Hank, and attempted to convince her daughter to get over her apparent insanity. However, her message of strength ultimately led Buffy to fight against the poison and return to save her friends in Sunnydale.
The First Evil also took her form to contact Dawn and tried to drive a wedge between the Summers sisters.
Years later, when cornered by the Scoobies, D'Hoffryn offered to grant them all one wish if they let him go; in Buffy and Dawn's case, he offered to resurrect Joyce and make her healthy again. While they were tempted, they ultimately refused, knowing there would always be a catch.
In the quiet year between the Supernatural Crisis Act and the Reckoning, Xander and Dawn had a baby daughter named after Joyce