Who is Justin Bieber and why does your 12-year-old love him?

If you are like David Cato, whose 12-year-old daughter Grace is unabashed in her love for teen singing sensation Justin Bieber, you might be scratching your head and wondering: "Why?"

"Everybody has their person," says Cato, of Atlanta. "She is into Miley Cyrus, but the Justin Bieber thing is at a whole new level. He has literally captivated the hearts of these little girls."

Is it his hair, that floppy updated Dorothy Hamill ‘do that he's famous for? Is it his angelic face and perfect teeth? His heart-throbbing lyrics in songs such as "Baby" and "One Less Lonely Girl"?

Well, it's all of those things, tweens say. But more importantly, Biebermania abounds because Bieber, 16, is seemingly accessible to his fans, both lyrically ("I can relate to his songs," says 12-year-old Caneel van Nostrand of Cobb County) and virtually, as Bieber has more than 4 million Twitter followers and tweets often with his whereabouts.

Now the Canadian-born superstar, who came to Atlanta to work with music manager Scooter Braun and is mentored by R&B star Usher, returns to perform in a sold-out concert at Gwinnett Arena on Monday. Don't have tickets? In December, Bieber will perform at the Philips Arena to the delight of thousands of "Bieberettes" hoping he might spot them in a sea of fans and profess his puppy love.

According to Bieber's interviews, they might have a chance.

"A lot of celebrities right now say they'll only date someone if they're famous, but Justin says he'd date a fan," Caneel confides.

Indeed, much of Biebermania may have less to do with his music and more to do with his peppy, playful persona, says 12-year-old Sara Howard, Caneel's gal pal in Cobb County.

"His music is good, but not phenomenal," Sara says.

Even Grace agrees, but hearts him anyway.

"He’s nice to his fans," explains Grace, who wears "I (heart symbol) Justin" T-shirts and signs a similar message at the end of her texts. "Fans really like him because he interacts with them and he appreciates the things they do for him."

That's a big reason she chats nonstop about him with her girlfriends, goes to his concerts, follows him on Twitter and Facebook, watches him on TV and YouTube, and, of course, has his syrupy-sweet face plastered across her bedroom walls. Surely she'll be among his fans who will read his upcoming book and see his 3-D biopic film rumored to be released next year.

Recently, Grace's Bieber-love hit a new level: She and a group of girlfriends collectively changed their Facebook names to his. Online, Grace Cato became Grace Bieber.

And that's where her dad stepped in.

"I said, ‘No, Grace, this has gone too far,' " recalls Cato, who says he's surprised his daughter is so engrossed with the teen celebrity. "She’s so bright, and she’s so anti-this, in my opinion."

But as Amber Rigsby will tell you, Biebermania can hit just about anyone. The public relations rep has gone public with her "underage crush."

"He was an acquired taste at first. I thought, who is this little kid?" Rigsby admits. "But before you know it, you are that 26-year-old girl driving down Peachtree Street turning up your radio because the Bieber fever hits you!"