In folk stories Kullervo is the son of the giant Kaleva. In the Kalevala however Kullervo is the son of a man called Kalervo.
In the Kalevala Kullervo's family is killed by Untamo's family before Kullervo is born. Only one woman is not killed. She gives birth to Kullervo who will be a slave of Untamo. Every work that Kullervo tries goes ill. This is why Untamo sells him to a smith called Ilmarinen.
Ilmarinen puts Kullervo to shepherd cows. Ilmarinen's wife gives Kullervo bread that has stones in it. While Kullervo cuts the bread his knife hits the stone and breaks. Kullervo gets angry and curses bears to eat cows and Ilmarinen's wife. Then he escapes.
While travelling Kullervo has sex with a girl. He does not know that this is his own sister. When he finds out he goes very angry and attacks Untamo and kills him and his family. Then he commits suicide.
The story of Kullervo differs from all other folktales in the respect that it describes the effects of child abuse in a very realistic way, and in the end of the poem Väinämöinen especially warns all parents from abusing their children.