Jillian was the personal secretary to the head of the Western Europe Watchers. In 1997 she announced her engagement to Monaco Area Supervisor, Dennis Gaetan. They plan to be married on 17 January 1997, her birthday.
Weeks before the event, however, Jillian is murdered by Immortal Jacob Galati on his anti-Watcher vendetta.
In a possible alternate timeline explored in the episodes To Be and Not to Be, Jillian fell in love with Watcher researcher, Adam Pierson. The couple was so completely in love, that despite the dangers posed by the Hunter takeover of the Watchers, he told her he was, in fact, immortal. Jillian accepted that, and eventually asked she be allowed to tell the other Watchers. She saw introducing a well liked Watcher as an immortal would heal the corruption within the organization: "Please. Please just let me tell them." He knew human nature better than she, however: "What are you going to tell them, Jillian? That I am an Immortal and I've been masquerading as a Watcher all these years? They're not going to like that. They are not going to throw me a party."
She persisted, however: "They know you. They like you. You'll make all Immortals human to them." The ancient cynics response, however: "Do you think that the Turks didn't know the Armenians were human? Do you think that Hitler didn't know the Jews were? To admit that they were wrong, they would have to admit that they were murderers. It's easier for them to make believe that we are all an abomination and just let the killing go on.... I love you, Jillian. More than I've loved anyone in so long. I can't remember. That's why I told you, but you have to trust me on this one."
Jillian, however, was determined to try and forge a peace. On the night he was to propose to her, she brought James Horton to their home, having told him of Adam's secret: "This is our chance to finally stop the killing." Horton and his Hunters had differing opinion and he shot her where she stood while his Hunters sprayed the rest of the room with machine gun fire killing Adam as well.
At that moment, Kronos, smashed through a window and shot Horton's killers, while Horton escaped. Methos revived to Jillian's dying request: "Forgive me."
It was her murder that was the impetus for the reformation of The Horsemen in that reality.