According to the "X-Men" star, it's the fact that she's remained so down-to-earth despite her intense level of fame.
"It's like that thing whereby you don't believe your own hype. Enjoy the good times, but don't get swept away with it. And I think that's something she's capable of doing and that's what makes her special, but yeah, it is very odd, and I am just kind of a bystander in many ways," he tells the magazine, via E! News.
The British actor, 24, also couldn't be more proud of her.
"It is well-deserved for her. I haven't really seen any change in her."
Nicholas and Jennifer, 23, met while filming "X-Men: First Class" in 2011, and briefly split in 2013 before starting up their relationship again.
According to Nicholas, the two have been having a blast being together while filming the sequel "X-Men: Days of Future Past."
"It's fun because in this business you are away from one another for long periods of time, so when you're on set together it's a brilliant thing, because you actually get to spend time together," he says.
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