- You may be looking for the character from the Human Nature novel.
Jac was a member of UNIT.
She investigated the mystery of how all the planes on Earth could have "frozen" in mid-air simultaneously. She said it was beyond any human technology, a conclusion she came to due to her love of science fiction TV shows. She was relieved when the planes were released by Missy, and then used a computer algorithm in order to search for the Doctor. The algorithm was based on searching for words such as "blue box" and "Doctor", which generated many results. Using criteria given to her by Clara Oswald, she was able to narrow it down to a single location, Essex in 1138. (TV: The Magician's Apprentice)
She helped investigating in the Zygon invasion that took place after a group of rogue Zygons called Truth or Consequences decided that a peace treaty with humans was unacceptable. She followed what she assumed was Clara Oswald to Clara's apartment, when they found a lift that went to some underground tunnels where Sandeep had been taken. Returning to the tunnels with UNIT reinforcements, Jac discovered the Clara she assisted wasn't in fact Clara, but a Zygon duplicate called Bonnie instead. On Bonnie's command, Jac and the UNIT soldiers around her were reduced to piles of dust. (TV: The Zygon Invasion)