Chell Blackstone (or Chell Blackstone Jr.) is the daughter of Frank Blackstone and Chell Johnson, who was born for the purpose to become an assassin to work for her father. Chell gave birth to her in 2011, although Frank managed to accelerate Chell Jr's aging to make her older. At present moment (2016) she is ten-years old. Despite Frank's plans for her, Chell Jr. has now fallen into the clutches of his enemy G-Woman, who plans to use the girl against her father and to benefit the Clandestine Company.
Chell fell pregnant to Frank Blackstone and gave birth to a baby girl. She initially thought it was the daughter of her presumably deceased partner Nathan Johnson, but Frank managed to manipulate the hospital so the baby girl had his DNA instead of Nathan's. Frank marries Chell, and names their daughter Chell Blackstone Jr.
Chell flees to Aperture Laboratories with the baby to escape Frank. Despite this though they come up against a bigger threat when an unstable core named Schizophrenic Core takes over the facility. Chell destroys him and escapes with the baby.
In December 2013 (two-and-a-half years since her birth) Frank accelerated Chell Jr's aging, bringing her up to six-years old and began to train her, hoping for her to become an assassin to work for him.