Heath Ledger and Mary-Kate Olsen first met over the summer of 2006, at the time he was living at the Chateau Marmont in Hollywood, PEOPLE reports.

"Mary-Kate and Heath were casually dating for three months before Heath's death," a source tells PEOPLE. "They were hooking up, but neither were particularly interested in making it exclusive."

They had a bond that was based on partying, and they had the same tastes in partying ... like, in terms of where they liked to hang out in New York, what time they would want to go out. They just had the same sensibility."

"Mary-Kate made him laugh a lot," an eyewitness recalled.