* Levinsen wrote all his numerous stories anonymously under the pen name of Tykke-Niels, that was also the name of the leading character from the beginning. From 1950 on the leading character was mostly Pillerik featured in other stories that were reminiscent of the Tykke-Niels stories.

The series premiered in 1935. Stopped in 1973.

The stories take place in a fictional suburban town in Denmark, and they feature a vast character gallery concentrated around teenage students and teachers of the local Latin school. The leading character is the goodnatured but slightly mischievous Poul-Erik Brun called Pillerik, who plays numerous pranks on the school's poor, unpopular mathematics teacher Julius Th. Mortensen known to the students as Sjakalen (The Jackal).
A few other characters are Pillerik's best friend Stærkodder, the antagonist Røde-Karl, the nerdy Alf Andersen, the girlfriend Jytte Staffeldt, and the dreaded toddler tyrant Buldermand.