STOCKHOLM—Felix Kjellberg doesn't play a very convincing megastar. The seemingly modest Swede avoids the spotlight, has no entourage and is uncomfortable with success.

But put Mr. Kjellberg in front of a Web-camera and he transforms into PewDiePie, by far YouTube's biggest draw. He has built a base of 27 million subscribers using a decidedly unorthodox approach to playing video and mobile games.

His videos aren't traditional game reviews. "Pewds," as he is often called, simply plays games and allows his audience—mostly teenagers—to peer in on his experience and hear random opinions interspersed with odd behavior. He contorts, screeches, swears, sings and even "twerks" to portray his feelings.

The 24-year-old Mr. Kjellberg, who created PewDiePie five years ago, has parlayed his persona into a brand name that pulls in the equivalent of $4 million in ad sales a year, most of it pure profit. In December 2012, PewDiePie signed on with Maker Studios, a producer of online content that takes a cut of ad sales. Maker Studios—which counts on PewDiePie as its most important personality—sold itself to Walt Disney Co. earlier this year in a deal that could be worth close to $1 billion, depending on certain performance targets.

His following is so big that even games he criticizes get coveted publicity.