Cold, humourless, and with complete disregard for alien life, Tilda Brennan was a typical Torchwood operative. She grew to loathe the freelance agent Jack Harkness and distrusted anything Torchwood gained through his involvement. For some time she was in contact with Bilis Manger, who manipulated her through her desire to get rid of Jack Harkness.
In August 1941, she was told by Manger to follow the instructions in File TW3/87/BM . She drugged two of her teammates, destroyed Turing's Rift predictor, and vivisected their alien prisoner "A. Neil". When Jack Harkness entered the Vaults to confront her, she was holding the seemingly blank diary of Gideon Tarry and had a gun to the head of Greg Bishop. She told Jack that she was no longer part of Torchwood and that everything she had just done was his fault.
During this standoff, the Light revealed it had been possessing her when her eyes shone with light and she spoke for someone else, saying that one day Jack would realise that (the other person) was just the messenger. As the Light left her, she briefly lowered her gun, allowing Jack to fire the two shots that killed her.