Explaining that Darke, 29, is "absolutely" his best friend, Radcliffe, 25, told Us, "I think that's the kind of relationship I always aspire to have with someone I'm in a relationship with. You want that person to be your best friend."
"In the case of Erin, we definitely are," the Harry Potter alum added.
Chatting with Us at IvyConnect and Cadillac's screening of Radcliffe's new film What If, the actor also reflected on relationships as a whole, and today's understanding of how dating begins.
"I don't think you can ever control how anything starts," the British actor said. "It's not like I seek out that way of meeting people or think I have to be their friend first but I think it's often a very natural thing to progress from into a relationship. The whole modern idea of 'friend zone' and all that stuff—that's a word I don't like very much."
Radcliffe and his actor girlfriend met in 2012, when they were filming the drama Kill Your Darlings, and have been together throughout the two years since.
A source told Us Weekly in June that the stars are "very happy," "inseparable," and "have great chemistry."