Jillian was recruited by the Watchers out of the University of British Columbia where she majored in business administration. She was graduated from the Watcher Academy, class of 1987, with a class rank of 6 out of a class of 79. She was an equestrian champion, and was fluent in English, French, and Italian.
From 1987 to 1988 she worked as an historian before receiving the following assignments:
1988 - 1991 Tresleem Thomas
1991 - 1993 Cecelia Holtzman
1993 - 1996 Seraphina
Under the cover of a personal secretary to the head of the Western Europe Watchers, she was stationed in Monaco. In 1997, she announced her engagement to Monaco Area Supervisor, Dennis Gaetan. They planned to be married on 17 January 1997, her birthday.
Weeks before the event, however, Jillian was murdered by Immortal Jacob Galati intent on his anti-Watcher vendetta.
In a possible alternate timeline explored in the episodes To Be and Not to Be, in 1995, 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."She persisted, however: "They know you. They like you. You'll make all Immortals human to them." The ancient cynic's 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 meant 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 opinions, however, and Horton shot her where she stood while his Hunters sprayed the rest of the room with gunfire, killing Adam as well.
At that moment, Kronos, smashed through a window and shot Horton's gunmen, 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.
Although Lisa Butler was not the only actor on Highlander to have played more than one character, she is one of the only actors to have played five separate characters: Jillian O'Hara in Not to Be; Genevieve Hill in Haunted; Melissa Berkowitz in The Colonel; Kenny's mother in Reunion; and a passing woman in The Innocent.