"Actually, I lost him somewhere along the red carpet," Cyrus told PEOPLE. "I don't even think that I'm here with him anymore."
The couple, who have been dating since they met while filming the movie last summer, were each accompanied by several family members, including proud papa Billy Ray Cyrus. They started off their red carpet stroll together only to be drawn apart as they conducted their own interviews.
Hemsworth, 20, appeared relaxed and confident, even solo. In fact, the leading man was probably calmer than his girlfriend because "he's seen [the movie] more than I have," Cyrus, 17, said.
"I'm actually more nervous to see it than he is, because he's seen it with all the reactions of the audience, and I haven't."
Despite any nerves, Cyrus appears adept at hiding those emotions – even from Hemsworth. "I don't think she gets nervous about many things," the Australian actor said. "She's about as cool as they come."
As for their relationship, "We got along really well the first time that we met, and we shot every day with each other, and we just seemed to get closer and closer," the handsome Hemsworth said.
Once the movie premieres are out of the way, Cyrus has some housekeeping duties ahead of her – for her new house. "I really haven't had a chance to buy anything for it yet," she says. "I'm a little bit of a slacker."