When Jerom was first introduced in the series he was merely a dumb brute from the
Stone Age. By order of duke Le Handru he beats up everyone without mercy. The only thing that can tame him is
anise, which the duke frequently feeds him with. Wiske discovers that Jerom also has a softer side when he plays with her doll,
Schanulleke, and is scared that she will die because she refuses to eat. Wiske makes her doll "eat" by drinking the milk Jerom prepared herself when he is not looking. This convinces Jerom that Wiske is nice and he is motivated to join their side instead, even travelling back to the 20th century with them, thus becoming the final main cast member of the series. He moved in at
Lambik's place, despite the fact they often bicker about. In the first albums Jerom behaves and dresses like an uncivilized caveman. In
De Knokkersburcht ("The Knokkers Fortress") (1953) he even literally behaves like a dog. After a few albums he gradually acts more polite towards others and starts to dress in modern 20th century suits. First by wearing a
tie above his animal skin in
De Tam Tam Kloppers ("The Tam Tam Beaters"; 1953), then gradually wearing sweaters, shirts, jeans and shoes. Jerom even obeys his own
ethics. He only fights for just causes and helps everybody, even opponents, when they are in danger. He can't be bribed and never betrays his friends, contrary to someone like
Lambik. Jerom also never fights animals and tries to control his powers. Only when he is put under a magic spell, hypnotized or drugged with
chloroform Jerom joins the
antagonists. Jerom is usually calm and level headed. When other people around him act aggressive or showcase other heavy emotions Jerom always remains restrained and master of the situation. He always talks in a
telegram style and shortens
Lambik's name as "Bik". Because of his brutish appearance and simplistic way of speaking other people, even his friends, often underestimate his intelligence. They treat him like a child or a simple minded person. Sidonia often uses Jerom to do difficult or tedious tasks for her, like cleaning the dishes. This behaviour is not entirely unjustified, seeing that Jerom is often not familiar with certain modern phenomena and needs to be explained about them. Despite being far more civilized than he was when he first appeared in the series Jerom still doesn't always feel at ease in modern society. In
De Malle Mergpijp ("The Silly Marrowbone"; 1973) he even goes through an identity crisis and briefly returns to his prehistoric ways. ==Relation with other characters==