Rachel was a mortal woman and the adopted daughter of Immortal Connor MacLeod. After the death of her parents during World War II, Connor adopted her and raised her as his own. As she grew, she became a close friend and confidant, as well becoming the secretary for and administrator of his shop, Nash Antiques.
Rachel was born in 1936 in Amsterdam, Holland to Jewish parents. In 1943, a Nazi raid killed both of her parents, forcing her to hide. She was found by Connor MacLeod, who defended her from an armed Nazi officer during an attack in which he was shot in the back. Although he had multiple bullet wounds, Connor stood up unaffected. Rachel saw this and would later realize it was a sign of his immortality. This was also witnessed by the soldier who shot him, and was perplexed how he was unscathed. MacLeod remarked that someone from "the master race" should understand immortality and returned fire, which immediately killed the SS commando.
Adopting her as his daughter, Connor brought Rachel to America to provide her with a decent life. Connor raised Rachel as though she were his own and the two formed a close father-daughter bond. 
In 1985, Rachel had long since become Connor's assistant at his shop, Nash Antiques. She later met Brenda Wyatt, a forensics expert who had grown suspicious of Connor. In order to hide Connor's identity, who had been living under the alias of Russell Nash, Rachel passed herself off as Connor's mother. Later, Brenda found out about Connor's Immortality and the two fell in love. Before going into his final battle with the Kurgan, Connor left Rachel in charge of Nash Antiques and said his goodbyes. Connor was successful in killing the Kurgan and eventually married Brenda. The couple soon after moved to London. Up until Kurgan's defeat and Connor's marriage to Brenda, Rachel's fate remained the same in all varying continuities. But in the TV Series continuity, which expanded on the story line, Rachel continued to play an integral part in Connor's life.
In 1952, Rachel graduated from high school, Connor recording a film of it to keep. In 1963, Rachel went to Europe with her lover, Richard. Things didn't work out, however, and the two broke up. At some point she met Connor's kinsman and fellow Immortal Duncan MacLeod.
The Dark Quickening
In 1987, after Brenda had been accidentally hit by a car, Rachel was called in and flew to London in to help Connor take care of her. Fortunately, Brenda was able to make a full recovery. She joined the two, as well as Duncan, on a picnic outside of London. It was there that they also noticed that Connor hadn't been acting himself recently. They would soon learn that Connor had been suffering from a Dark Quickening after having taken the Kurgan's head. Connor was kidnapped by Immortal Gordon Byrne, who wanted to see how strong Connor's Dark Quickening had become. Connor was rescued by Duncan, but Byrne came to Nash Antiques to try and attack Rachel and Brenda. Duncan was able to save them both, but Byrne escaped. With the assistance of Duncan and Duncan's Immortal friend, Hugh Fitzcairn, Connor was able to stop his Dark Quickening and return to normal. Afterwards, Connor was able to take Byrne's head. With Connor's senses regained, Duncan left Connor to be tended to by Rachel and Brenda.
In 1994, Rachel returned home to see the television was on playing old videos of her and Connor from the time when she was still a young girl. She then answered a ringing phone which turned out to be a bomb. The explosion destroyed Nash Antiques, killing Rachel in the process.
Rachel's death devastated Connor, especially after having lost Brenda, Alexandra Johnson and John MacLeod so recently. Connor quickly retired from The Game and hid in the Sanctuary. A decade after her death, Connor would learn that Rachel's killer was Jacob Kell, an Immortal with a personal vendetta against Connor. Kell was later killed by Duncan, who had been forced by Connor to take his head in order to gain more power. With Kell's death, Rachel had been avenged.
Rachel was a strong voice of reason in Connor's life, being one of the very few who could talk to the Highlander on an even keel. She was strong-minded, and completely accepting of Connor and his "kind of magic." In Connor's eyes, however, she was also "a romantic" - perhaps hopelessly so.