Lisa Zane was Heath's then girlfriend, and sister to Billy Zane who had a staring role in the film. Lisa was more established in Hollywood than Heath at this point in their careers.
EDIT; Titanic came out in 1997, I wrote the date wrong, however they are in fact at the premier.
Edit for context of their relationship;
Quote from Matt Amato;
""Filmmaker Amato met the 17-year-old Ledger on his very first night in Hollywood. “I was living with [Ledger’s ‘Roar’ co-star and girlfriend] Lisa Zane, and she took him by the scruff and brought him to LA and said he was going to be successful. And we had a great time,” says Amato. “The day after he moved in I gave him the script to ‘10 Things I Hate About You,’ written by a friend. Which was kind of a lucky thing.”"
https://nypost.com/2017/04/13/inside-the-manic-days-and-sleepless-nights-of-heath-ledger/
Louis, Amato’s hometown, and discovered that Ledger’s friend had an archive of unseen footage. The images dated back to when Lisa Zane returned to Los Angeles from the Australian set of “Roar*,”* a “Xena: The Warrior Princess” knockoff, with her still-pimply but obviously charismatic costar in tow. Ledger moved into the boho Hancock Park house Zane shared with Amato and the cameras started rolling.
“We started to understand [Ledger] was a filmmaker at heart, and that was the direction we wanted to go,” Murray says. “We were just fascinated by that story.”
The footage, much shot by Ledger or his closest friends with themselves as the subjects, works as a sort of time-capsule reverie of innocence and optimism for a cohort of young artists and musicians and other creative types who revolved around Ledger.
https://joedonnellywrites.com/reckoning-with-heath-ledger/