The music mogul revealed that Spears, who signed a $15m (£9.6m) deal to join the Fox singing competition's judging panel in May, has a "shy" side that fans may not expect.
"She's not on the show to be Britney Spears on a TV show," Cowell told People of the 'Criminal' singer.
"She's actually there to prove to the world there's more to her, and she's got the ability to do this job really well. She's taken the role really seriously."
> 'X Factor' USA's Simon Cowell: 'The Voice knows that it stole from me'
Cowell continued: "It's quite interesting because when someone has done what she's done as she has with her career, and being such a public person, you kind of think she is going to be very confident, but in real life she's shy.
"There's kind of a mystique about her which I think is what kind of attracted me to her in the place."
He recently dubbed Spears and fellow X Factor judge Demi Lovato as "the gruesome twosome", joking that Lovato was "more annoying than ever" and Spears had "grown claws" during his absence from the production.
Watch a promo for The X Factor USA below: