Rachel Blake (born Rachel Dunham) is Olivia Dunham's younger sister and the mother of Ella Blake. She comes to visit Olivia from Chicago after she suffers from marital issues with her unseen husband, Greg.
Eventually, Rachel moves back to Chicago where her husband finds a new job, implying they may have reconciled off-screen. In the new timeline, Rachel and Greg either reconciled sooner or never faced the same marital issues witnessed in the original timeline. They had a second child, Eddie Blake.
Rachel is the youngest daughter of Marilyn Dunham. She is at least four years younger than her older sister, Olivia. At around 9 or 10 years old, her mother died and she was brought into foster care with Olivia.
Soon after Olivia Dunham had escaped captivity by the hands of Mitchell Loeb, she receives a phone call from her sister Rachel, who needs a place to stay, because she is going through a rough patch with her husband, Greg Blake. Olivia agrees, and Rachel and her daughter Ella Blake moved in with Olivia, though they claim it is only temporary. ("Bound")
Rachel adapts well to living with Olivia. However, one day Ella uses Rachel's computer when she is attacked by the brain-melting computer virus which nearly kills Ella, without the intervention of Peter Bishop and Olivia Dunham. Rachel later observes Peter and Ella playing together, and seems to be attracted to him. When Peter later arrives to speak with Olivia, he and Rachel share a look that alerted Olivia to the chemistry between the two. ("The No-Brainer")
Rachel's husband Greg Blake later files for divorce. Though Rachel expects this, she does not expect Greg to file for sole custody of Ella Blake, which Greg did, on the grounds that Rachel was an "unfit mother." ("Midnight")
After Olivia Dunham was in a car wreck, she informs Peter Bishop that Olivia had a living will. ("A New Day In The Old Town") Rachel leaves Ella with the Fringe Team as she handles personal affairs in Chicago. She is last seen in the series saying goodbye to Olivia, worried about what job she has to do, unaware she is crossing over to the Other Side.
Rachel moves back to Chicago off-screen and lives with her husband who has found a new job there. It is implied they reconciled. ("Amber 31422")
In the future of 2026 (the end of the original timeline which will never happen due to the re-written timeline), Rachel and her husband are nowhere to be seen, as Ella seems to be under the custody of Olivia, and even shares her mother's maiden name. It is implied that both Rachel and Greg have died, as neither are seen at Olivia's funeral nor the aftermath where Ella seems to be staying and taking care of Walter. Ella also claims that she doesn't remember much "before it got bad", a possible nod to her parents' fate in the dying world of the future. ("The Day We Died")
Rachel does not appear in the new timeline, but details of her history have changed significantly.
In the new timeline, when Olivia's memories began to get overwritten by her memories of the original timeline, she inaccurately remembers their separation and forgets the existence of her nephew. This was used as grounds to attempt to have her removed from active duty and declared mentally unfit.
Olivia inaccurately refers to Greg as Rachel's ex, who she spitefully calls a "bastard" and does not remember Rachel's second born child, a son named Eddie (named after her grandfather).
It is unknown how Peter's erasure changed the course of Rachel and Greg's relationship. It is possible that because Nina Sharp raised Rachel and Olivia, her different upbringing possibly shaped her into having a better marriage life. The other possible and more likely explanation is Rachel becoming a close confidant of Peter in the original timeline, where she was even flirting with him. This may have changed the course of events, as Rachel implied Peter would be a good father figure to Ella in Greg's absence. Because Peter was no longer there, she would have returned home sooner to Greg and fixed her marriage.
Although Rachel remains unseen in the new timeline to show this. Olivia says that while they're close, they don't talk much due to the nature of her work. ("Nothing As It Seems")
"Go, save the world."
- - Rachel to Olivia (Bound)