Karlsson is a very short, plump and overconfident man who lives in a small house hidden behind a chimney on the roof of "a very ordinary apartment building on a very ordinary street" in Vasastan, Stockholm. When Karlsson pushes a button on his stomach, it starts a clever little engine with a propeller on his back, allowing him to fly. In his own opinion, Karlsson is the best at everything. He befriends Svante Svantesson, a seven-year-old boy and youngest member of the Svantesson family (who is often referred to as "Little Brother", Swedish: Lillebror, or "Malysh" in the Russian adaptations). Svante is sometimes called Smidge in the US version of the books. Karlsson is quite mischievous and likes to make fun and prank others. He often gets Lillebror into trouble, as Karlsson usually disappears just before Lillebror's family arrives leaving him to deal with consequences of Karlsson’s actions. At first, parents, siblings and friends of Lillebror don't believe that Karlsson is real and consider him being an imaginary friend but after they meet him in person they begin to like the little flying man. Another character to encounter Karlsson is Fröken Bock (Miss Hildur Bock), a mean nanny (presumably in her late 40s or 50s), who undergoes an emotional transformation after meeting Karlsson.