Tilda Brennan
Doctor Matilda B Brennan was the head of Torchwood Three in 1941.
Profile
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 an unknown amount of time she was in contact with Bilis Manger who manipulated her through her desire to get rid of Jack Harkness.
In August of 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 realize that (the other person) was just the messenger. As the Light left her, she briefly lowered her gun allowing Jack to shoot the two fatal shots that killed her.
When Rhydian and Jack placed her body on Tray 78 of Torchwood's morgue, Jack slipped the diary in with her, where it remained until 2009.
Note
Its not known when Tilda Brennan became possessed but her actions were likely her own when Bilis was manipulating her. Only The Dark has shown the ability to either subtly manipulate or convincingly mimic the person they were possessing but even they didn't display this ability by 2009.
