The Rebeccas are the two twin girls in the leading Styx family, and the main antagonists in the Tunnels series. Their father is the Crawfly, and their grandfather is the Old Styx. The Rebeccas originally pretended to be Will's Topsoiler sister. The twins plan to release the Dominion virus and have the Styx reclaim Topsoil. They have been hunting down Will in revenge for trying to stop the Styx and killing their father.
When they were two years old, the Rebeccas were placed into Will's Topsoiler family. Since they couldn't be on the surface at once, they took turns; one Rebecca would be below ground with the Styx, the other would be Topsoil with the Burrows. They tried to flush Sarah Jerome into the open, and prevented her from bringing about the Discovery of the Colony. The surface Rebecca would run the family; she would pay bills, wash dishes, and pretty much do what Dr. and Mrs. Burrows should have been doing.
In Tunnels, Rebecca is introduced as the powerhouse of the Burrows family. She seems to not really care about Will and Chester's digging. One night, she sees someone rummaging around the Burrows house (possibly her twin coming to relieve her). Will tells her about his encounter with the pallid men chasing him and Chester, and Dr. Burrows's journal, which described his investigation of these men. He doesn't tell her about the tunnel under their house. At the end of Part 1, Rebecca is sent to live with Auntie Jean.
Rebecca hates the conditions at Auntie Jean's apartment. She decides to leave for the Colony, putting on her Styx garb and destroying Jean's cigarretes and vodka. She then appears as Will, Cal, and Chester are escaping to the Eternal City. She reveals she is a Styx, and lets the Styx recapture Chester. Once Chester is back in the Hold, she comes to watch him being beaten with clubs. Later, she appears at a Styx Panoply meeting, where she yells at Tam for not responding to a question. It is revealed she is the Crawfly's daughter. In the epilogue of the book, Rebecca goes Topsoil to poison Imago and kill him, for letting Will and Cal escape.
Rebecca, the Styx, and some Colonists catch up to Sarah Jerome, who is hiding with Bartleby in Will's underground base. Joe Waites appears and tells Sarah that it is true that Will killed Tam. Sarah tries to commit suicide, but manages to stop herself. Rebecca and the Styx take her back to the Colony using an elevator. Once in the Quarter, she, the Old Styx, and his young assistant take a carriage with Sarah to Grandma Macaulay's house. Sarah is delighted to meet her mother, but in fact, she has been tricked into having Sarah hunt down Will. Rebecca and the Old Styx discuss how well the ruse is going, and then plan to kill Joe Waites and Grandma Macaulay.
Rebecca then appears with the Old Styx on Admiralty Arch. Her grandfather compliments her on how she recaptured Sarah and started to purge the Deeps of renegades. They then release doves containing a reduced strain of the Styx's Dominion virus (the Ultra Bug). She returns to the Colony and has Sarah prepare for her trip to the Deeps to find Will and Cal. Once on the train, Sarah slowly learns about the Dominion plot through a newspaper Rebecca was reading (Rebecca was commenting on how well the plan was going).She is also given the Dominion virus and vaccine from the Old Styx. Rebecca, the Styx, and the Limiters manage to follow Will and Co.'s trail using Bartleby. She then has the Limiters search around the subterranean sea for Will, but Elliott's charges blow the island up and kill them. Rebecca finally appears at the Pore, where Will and his friends are trapped. She, the Styx Division, and the Limiters are surrounding them. Rebecca soon reveals there are two of her, and they tell Will about the Dominion virus. Refusing to turn himself in, they orders the Styx troops to fire. They kill Cal and cause the others to fall into the Pore. The Rebeccas think they all died falling in, but Sarah, who realized what the Styx did to her, flings herself at the twins, causing them to fall into the Pore.
The Rebeccas survived the fall into the Pore by landing on one of the fungal outcrops; however, they had many injuries. One of them broke her teeth, and the other was lame and had her arm pulled out of its socket (this was fixed). They see the bottle rockets launched by Will and Chester, and then meet up with the two Limiters that jumped into the Pore. They form a plan; one Rebecca will find Will and give him the Ultra Bug phials to trick him into thinking it is Dominion.
- The Rebeccas are refered to as the "Evil or Foul Cows" by Will and Chester.
- The Rebeccas reveal that there are two of them in Deeper.
- Uncle Tam killed The Rebeccas' father, the Crawfly, and have hunted down Will's family ever since, and anyone who has helped him.
- The girls' favourite song is 'You Are My Sunshine', which ironically, Will Burrows hates.