The key to their below-the-radar relationship, he tells PEOPLE, is that they both love to have a good time.
"That's the thing between me and her, we just like having fun," he said Sunday at Las Vegas's Lavo club, where he was celebrating his 34th birthday.
Although Ashani was in Japan and couldn't attend the party, Nelly admitted that the two have been "hanging out for a minute." (That's a bit of an understatement. In fact, they've been linked on and off since 2003.)
So is it serious?
"I think it's always serious,” he said, “when you can hang with a person continuously ... and [“still”] have fun."
Earlier this summer, Ashanti would only call Nelly a "good friend." But, she said, an engagement could "definitely [be] in the future."