Keryehla Janees, aka Beth Halloran, rank Putaak Graszh, serial number ish-nin-fas-du-hap-vac-nal, the wife of Mike Lyndon, was a member of Cell 114 disguised as a human. She believed herself a human until informed otherwise by Torchwood 3.
In the 2000s, Keryehla Janees was sent to Earth as a sleeper agent to gather information about the planet, and was given a false personality, false memories and the false identity of Beth Halloran. Keryehla, as Beth, at some point fell in love with and married Mike Lyndon.
When two burglars invaded Beth and Mike's flat at 114 Brodsky Street in the night and knocked Mike unconscious, Keryehla's real personality temporarily took over and attacked the burglars as self-preservation. Keryehla killed one of the burglars and badly injured the other, before the Beth persona reasserted itself and Beth lost memory of attacking the burglars.
After Torchwood Three discovered that Beth was the one who had attacked the burglars, she was taken to the Hub to be interrogated. Beth was completely unaware of what she really was due to her false memories and persona, but Torchwood realised otherwise due to numerous phenomenons surrounding Beth; lights overloading when Jack Harkness' harsh interrogation upset her, Beth's force field causing needles and even knives to snap when Owen Harper tried to take her blood sample, and the Weevil Janet reacting with fear when Jack showed her to Beth to prove to the latter that aliens existed.
The Torchwood team used a mind probe on Beth which temporarily unlocked her Keryehla persona and activated her Cell implant. Afterwards, the team explained this to Beth, and decided to deactivate her implant and put her in cryo-stasis until in future they could find a way to stop Beth's Keryehla persona from activating. However, Beth's Cell abilities allowed her to fool Torchwood's equipment and avoid cryo-stasis, and she then left the Hub to say goodbye to Mike at the hospital.
While saying goodbye to Mike, Beth's arm turned into a blade while she was hugging him, badly injuring him. Jack and Gwen Cooper then arrived to take Beth back to the Hub, when they discovered that three other Cell 114 agents had activated and were starting the invasion plan. Jack and Gwen got Beth to try and track the other agents, and Beth used her implant to track the only remaining agent down and stop him.
Afterwards, Beth was taken back to the Hub, and the Torchwood team prepared to refreeze her. However, Beth pretended that her original personality had activated and pretended to try and kill Gwen, successfully tricking the rest of the team into shooting and killing her. Knowing that it was only a matter of time before her Beth persona would be erased and that being frozen was just a way of delaying it, she chose suicide to save Torchwood the pain of finishing her off later. (TV: Sleeper)