Kreepo was originally meant to be Kalus' companion throughout the game and had a fierce rival with Meepo. However, Meepo was removed from the game, and thus Kreepo's role lost its purpose so he was repurposed as the common squealing dragons that are seen flying around Thorn's Castle.
Kreepo helped Kalus and even fought for her.
Kalus would cast spells on him to make him grow into a giant flaming version of himself. Not much is known about Kreepo other then he has a squinting eye and displayed a serious type of character.