King K is a KP Koopa which is basically a yellow version of a Koopa Troopa.
Unlike most characters in the Glitz Pit who are usually rude and competitive, King K is very friendly and doesn't show any hostility toward Mario. King K is also very good sport who isn't bothered by his loss to Mario unlike most of the others.
When Mario, under the name of The Great Gonzales, first entered the minor league of the Glitz Pit, King K greeted him and introduced Mario to the other competitors. King K will commonly talk to Mario after he as won every match he is in. Right before Mario fights the Punk Rocks, King K tells him that he is thinking about retiring. After Mario defeats Master Crash and the Bob-omb Squad, King K is gone with the others, except Cleftor, missing him.
Eventually, it was later revealed that King K saw Grubba using the draining machine and Grubba drained his energy from him. King K was later restored to himself and competes in the Glitz Pit again. Bandy Andy of the Hand-it-overs has the same happen to him.