Jac (The Magician's Apprentice)

Jac was a member of the Unified Intelligence Taskforce.

You may be looking for the character from the Human Nature novel.

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 decided that a peace treaty with humans was unacceptable. She went side-by-side with Clara Oswald around England to find out what exactly was going on, until she found that the Clara she assisted wasn't in fact Clara, but a duplicate instead. She was killed by Zygons along with a group of UNIT soldiers. (TV: The Zygon Invasion)