Billboard Cover: Ariana Grande on Fame, Freddy Krueger and Her Freaky Past Billboard (Ft. Ariana Grande)

The “micromanaging workhorse” finally achieved her dream of becoming an R&B diva. Now, at 21, comes the hard part: conquering her fear of futuristic pop.

Ariana Grande is easily mistaken for an old-school good girl. The singer wears Audrey Hepburn-style strapless gowns, travels everywhere with her mother and trades lovey-dovey messages with on-again, off-again boyfriend Jai Brooks (of Internet comedy boy-troupe The Janoskians) over Instagram. Last year, she implored fans to boycott SeaWorld after the damning documentary Blackfish came out.

"I think people see me as a little cutesy thing," she says, looking, in fact, demure and adorable, seated with her feet tucked underneath her on a giant leather couch. "But I’m literally the most sardonic person you’ve ever met."

Proud oddball Iggy Azalea -- Grande’s collaborator on "Problem," the song-of-the-summer contender that has elevated Grande to superstar status -- allows that Grande, while "very sweet," is definitely "quirky." Last year, bloggers simply had her pegged as a "mini-Mariah" for the lush, unabashedly ’90s R&B sound of her debut Yours Truly, which went to No. 1.

On Aug. 25, Grande presents her follow-up, My Everything, and three huge singles released in the run-up to the album have already redefined her as a state-of-the-art pop diva: Besides "Problem," a super-catchy, not-at-all-smooth, kiss-off track, there’s the Ibiza-ready "Break Free" and the woman-power anthem, "Bang Bang," with Nicki Minaj and Jessie J.

Grande, who now lives in Los Angeles, says she was "a very weird little girl" growing up in Boca Raton, Fla.: "Dark and deranged. I always wanted to have skeleton face paint on or be wearing a Freddy Krueger mask, and I would carry a hockey stick around. I was like a mini-Helena Bonham Carter."

Sitting in the cavelike lounge we’ve retreated to in downtown L.A., she looses a throaty, almost maniacal laugh. "For my fifth birthday party we had a Jaws theme and all my friends left crying. I mean, I still am that way. But when I was little it was more concerning. There was a stage, when I was 3 or 4, where my mom thought I might grow up to be a serial killer."

Instead, Grande embarked on a show-tunes-inspired career. She started acting in community theater a few years after the creepy birthday party, and by freshman year in high school, she auditioned and scored her first casting in a Broadway show, 13. That led to a supporting role on Nickelodeon’s teen musical sitcom Victorious and a starring part in the Victorious spinoff, Sam & Cat.

Republic Records chairman/CEO Monte Lipman signed Grande when she was 17 and little-known beyond Nickelodeon, after a friend excitedly sent him YouTube videos of her covering Whitney Houston and Mariah Carey. Sam & Cat only lasted one season; the final episode aired in July, amid much drama. But two weeks later, "Break Free" placed No. 1 on iTunes.

Making a hit record was always the goal. "I remember when I first came to L.A. to meet with my managers, I was like, 'I want to make an R&B album,' " recalls Grande. "They were like" -- she drops her voice a few registers --" 'Um, that’s a helluva goal! Who is going to buy a 14-year-old’s R&B album?!'"

Grande’s restless ambition is accompanied by a twitchy energy: She’s hummingbird-tiny at just five feet tall, often walking with a stutter step thanks to her giant high heels.

She makes dramatic arm gestures as she talks, with the words tumbling out of her mouth. "I am hypoglycemic so sometimes I’ll get anxious if I forget to eat," says Grande. "When I was a little girl, I would turn into the Tasmanian devil."

"Because she is a perfectionist, the one thing I’ll say to her every now and again is, ‘Ari, perfect is not always about being perfect -- it’s those flaws that people can relate to,'"says Lipman. "I don’t want you to get to the point where you’re gritting your teeth and your fists are all balled up."