Karin Berquist (played by Melinda Culea) was a specialist in canine behavioral studies who befriended Mulder in an online chat room. After the arrival of the T'ien Kou and a number of brutal murders resembling dog attacks, Mulder sought her aid in the investigation. He and Scully visited her home, where she lived with many dogs and in near total darkness with all the blinds closed. She published a number of books about canine behavior such as Better Than Human.
Scully correctly picked up on an attraction between Berquist and Mulder, thinking that each saw the other as a kindred spirit. While speaking privately with Berquist, Scully realized that she suffered from lupus, which would explain the perpetual avoidance of sunlight.
During the investigation of a supposed wanshang dhole, she encountered and disliked Ian Detweiler. Ultimately, she faced down Detweiler in the form of the wanshang dhole, which she had prepared to tranquilize but did not. As the creature lunged at her, they both fell from a second-story window. Berquist was killed in the fall. (TXF: "Alpha")