Harold has a gray mustache and wears glasses. He is light brown with dark brown holes. He wears a white shirt, brown pants, a black and yellow belt, and a tall white hat with a brown stripe. His head, along with his wife's, is much rounder in appearance than SpongeBob's, making them look more like actual sea sponges.
Harold usually seems to be a happy cheerful fellow who likes to take care of SpongeBob with his wife Margaret. He can also be a bit protective of his wife, as seen in "New Digs." He does, though, seem to like to spend time by himself, as seen in "BlackJack."
According to SpongeBob's thought in the episode "BlackJack," Margaret says "...it's so nice to spend quality time with you in the office all hours of the day," implying that he has or had, at one point, an office job.
He currently lives with Margaret since SpongeBob moved out.
According to Hooray for Dads!, SpongeBob gets his jellyfishing skills from Harold.