Harold is very smart and tech savvy which he tends to be arrogant about, being able to design and build many great machines even though they don't always work out. He becomes even more skilled at building under the influence of a meteor shard.
While he at first appears to be helpful towards his uncle Mayor Humdinger, he later shows himself to be just as greedy, wanting all the power of the meteor for himself. He is treacherous and frequently backstabs Mayor Humdinger to further his own goals. Despite this, Humdinger still trusts him.
Like his uncle, he's quite show-offish and egotistical. In "Mighty Pups: Super Paws: Pups Stop Harold's Big Freeze", he planned to carve out Adventure Bay from the mainland to create an island of his own to rule and rename "Harold Humdinger Island".
Harold has fair skin, dark brown eyes, and golden blond hair. He wears a purple T-shirt with the Kitten Catastrophe Crew logo on it, a dark belt, light gray long pants and purple tennis shoes. He has a gap in his teeth, similarly to Danny.
In his villain attire, he wears a purple helmet and armor with yellow lighting.
Here is a gallery of Harold Humdinger’s attires: