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Among the memories that linger: John's endearing habit of climbing the balcony railing to reach her apartment, then whistling to say he was there. The day he confided, "I can't imagine marrying anyone but you." And their first, impossibly romantic kiss, shared while they were playing lovers in an Off-Broadway play in 1985. On a moonlit night at his mother's New Jersey estate, the 35th President's son pulled his old friend close and whispered, "I'd like to kiss you for real this time." Remembering it, Christina Haag says, "I thought, 'This kiss might change my life forever.'"

It did. As chronicled in her new memoir, Come to the Edge, the moment marked the beginning of a passionate five-year relationship with John F. Kennedy Jr.-a relationship that helped define her and a love so strong that she "had a good cry" at the news of his wedding to Carolyn Bessette in 1996. When the plane he was piloting went down off Martha's Vineyard in 1999, killing him at 38, along with his wife and her sister Lauren, Christina, now 50 and an actress living in Manhattan and L.A., poured out her grief in journals. Those, along with diaries she'd kept since girlhood, formed the basis of a book that offers a rare glimpse into the privileged, freighted life of the Kennedy dynasty's favorite son. "It captures the person I knew," Christina says. "And that wasn't the person who was written about."

The two first met in the '70s, when Christina, the daughter of a marketing exec and a former model, was traveling in a wealthy Manhattan prep school circle that included John. He was fun, she recalls, "but a little awkward." And mischievous. One night at his Fifth Avenue apartment when mom Jackie wasn't there, Christina remembers, he and his pals threw Noxzema-filled balloons out the window at passersby.

She had no desire to date him-at first. "Not because of him," she says, "but all that went with it." Both attended Brown University, where they bonded over a love of theater and lived together as friends in a group house, along with three other roommates, including now-ABC anchor Christiane Amanpour. Their coming together, when it finally happened, was worth the wait. "To fall in love with your friend? As he said, it was very heady," Christina says. John wooed her by buying a motorcycle to ferry her from play rehearsal (he didn't care when it was stolen from him; he just bought it to impress her) and reading Seamus Heaney poems to her by the campfire on wilderness treks. "He had a magic way about him," she says. "He made you feel alive."

He sometimes spoke of his father, who was killed when John was almost 3. "He remembered the sound of the helicopters, which meant his dad was coming home," Christina says. She grew to feel "simpatico" with Jackie, with whom she shared a love of ballet and books. (Per John's instructions, she always addressed his mother as Mrs. Onassis.) And Mom didn't bat an eye when the couple shared a bedroom. According to John, she thought that "his grandmother Rose's attitude toward sex had created problems for his father, and she didn't want that for [John]," writes Christina.

John was happiest outdoors, but his appetite for risk sometimes scared his girlfriend. During a 1989 trip to Jamaica, he steered their kayak into uncharted waters, where they were swallowed by a giant wave. "We could have died," Christina told him. His response, which haunts her still: "But what a way to go."

Things turned serious by the late '80s, when John was studying for the bar exam, which he famously failed twice. ("He cried because of those headlines," she says.) Yet there were warnings that he wasn't ready to commit: a gossip item linking him to actress Darryl Hannah and then his request for more time. During one talk about fidelity, he confessed, "Sometimes it's like I fall off the wagon." They broke up in 1990 but kept in touch for a few years. "It wasn't lack of love," says Christina. "It was timing."

She went on to appear in episodes of ER and Heroes, battled breast cancer in 2004 and fell in love again more than once, but she hasn't married. "I'm a romantic," she says, "but marriage isn't important to me. Love is."

Going public with her memories after all these years wasn't an easy decision. Like most in John's circle, she'd always been protective of him. "I was one of those people who didn't talk," she says. While she didn't ask permission, she did write his sister Caroline, for whom she has "great respect and fondness," to let her know a book was coming. The response? "If there was a problem, I would have heard from her," Christina says.

In the end, she went ahead because "it's not a biography, it's my story," she says. "Love binds you to someone-you carry pieces of them in your heart. It was good and healing to remember."