He was not strong, or particularly knightly. Some sources say he was a hunchback, and lame, and cross-eyed, and a whole bunch of other things; since most of those chronicles were written under a rival branch of the dynasty, they probably need a grain of salt. Whatever the case, Kálmán (originally intended to become a priest) was more nerdy than kingly. His learning and intellect made him a very efficient ruler. One of the things he is known for is his law concerning witches: "There shall be no talk of witches (striga), since they do not exist." Most people see this as a sign of an enlightened ruler, but it is also possible that he only tried to strengthen Christian beliefs against pagan superstition.
The low point of his character was his attempt to secure his lineage against the succession of his stronger brother Álmos and Álmos' son Béla; he had them both blinded so they would be unfit to rule. He also wanted to have the child castrated, but the soldiers didn't go through with it.
(Béla did end up becoming king, known as Béla II the Blind, and ruled for ten years) (Kálmán did not see that coming).
The chronicle says that at the end of his life Kálmán fell gravely ill; he trusted an Italian doctor called Draco to cure him (although the name should have been a red flag). The doctor put a poultice on the king's ear for his ear ache... But it proved to be a tad too strong for the task, for when it was taken off, part of the king's brains came out through his ear with it. Oops.
This goes to prove that medieval medicine could kill you just as fast as the disease. Sometimes even faster.