Phoebe Green was a Scotland Yard agent in England.
Green was romantically involved with Mulder during their tenure at Oxford. She cheated on him, however, leaving him deeply hurt. Her relationship with Mulder ended and he returned to the United States of America, following the completion of his studies.
Assistance with Fox Mulder and Dana Scully
Phoebe Green was reunited with Mulder ten years later, when she came to America with an aristocratic British family who were being targeted by an arsonist. She reacquainted herself with her former lover by leaving a cassette tape in Mulder's car, unbeknownst to Mulder. When Mulder and Scully played the tape (now FBI agents and partners in the X-files unit), it stated that they had just activated a car bomb. Green unexpectedly threw open the car door, startling both Mulder and Scully, neither of whom seemed particularly amused by the joke. Though surprised by her appearance, Mulder did not appear pleased to see her. Scully took an immediate dislike to her, showing slight signs of jealousy thereafter, having deduced that Mulder and Phoebe had a romantic relationship in the past (though she would not admit this jealousy to anyone).
Green then requested Mulder's assistance with her case, explaining that several prominent British men had been killed in mysterious fire-related murders. Mulder said he would put in a request for the FBI's arson team to help her, but gave the impression that he would not personally investigate the matter; Scully seemed sure that Mulder would look into the case himself, though.
During the course of the investigation, Green tried to reignite some of the spark she once had with Mulder. She expressed regret at some of her past actions, referring to them as "youthful indiscretions," but also reminded Mulder of an encounter they shared on top of Sir Arthur Conan Doyle's grave. The family she was watching over attended a banquet at the Venable Plaza Hotel in Baltimore, and Green pointedly told Mulder she had taken a room in the hotel that the event was scheduled in, implying she wished for the two of them to use it together. Mulder similarly decided to attend the banquet. While there, Green and Mulder engaged in a slow dance and then shared a gentle kiss, which was witnessed by an annoyed Scully, who had come to the hotel to advise them that she thought she had discovered who the culprit was: a young Irish man named Cecil L'Ively.
L'Ively started a fire on the fourteenth floor of the hotel, which was where the children of Green's charge were located. Mulder attempted to rescue them, but was unable to do so because of his pyrophobia. The children were rescued by none other than L'Ively, who was posing as the family's handyman and assistant driver. While Mulder recovered in his hotel room, Scully told him of her theory. He went to the family estate to help in speeding up their return to England. Upon his arrival at the house, he interrupted Green and the husband from a romantic interlude, and this destroyed any further feelings he had left for her. Scully then realized that L'Ively was in fact a man she had seen at the hotel, and she went to the house to tell Mulder who he was. Mulder discovered that L'Ively possessed pyrokinetic abilities, which is how he had caused the burning deaths of the previous men. L'Ively used his powers to trap Mulder in a burning hallway and then attempted to make his escape, but he was incapacitated when Green tossed a flammable liquid into his face. Mulder was able to rescue the children, and they all watched as L'Ively was immolated (but not killed) on the house's front lawn.
Following these events, Green returned to England with the family without bothering to confront Mulder, apparently ashamed of her actions. She did leave another pre-recorded cassette for him, but he did not to listen to it, deciding to put his past with her behind him. (TXF: "Fire")
The invention of an English former spouse of Mulder's was simply something Chris Carter wanted to do. "I thought it was interesting to show a little bit of Mulder's history by bringing an old girlfriend back," Carter said. "I've always wanted to do a Scotland Yard detective who was a woman." (Cinefantastique, Vol. 26/27, No. 6/1, p. 41)
Phoebe Green was played by Amanda Pays. Chris Carter was intrigued by the possibility of casting her in the role. "I just thought it was an interesting chance to use Amanda Pays," he remarked, "and to make a villainess out of her." (Cinefantastique, Vol. 26/27, No. 6/1, p. 41)
Phoebe Green was originally considered as a possible recurring role, owing to Chris Carter's admiration of Amanda Pays, his love of Sherlock Holmes and the fact he was intrigued by the idea of incorporating a Scotland Yard detective into The X-Files. (The Truth Is Out There: The Official Guide to The X-Files, p. 129) In fact, Pays was briefly considered by the Fox network as a replacement for Gillian Anderson in the role of the series' female lead actress, unbeknownst to members of the creative staff such as Glen Morgan. (The Complete X-Files: Behind the Series, the Myths and the Movies, p. 43) Though the character achieved the desired effect by becoming a role that, according to Carter, "fans on the internet loved to hate," he believed that the chemistry "didn't work as it might." (The Truth Is Out There: The Official Guide to The X-Files, p. 129) This explains why Phoebe made no further appearances after the first season episode "Fire".