Harold "Bill"  "Scott" Reginald is a blue fish and like other character models, his appearance is not perfectly consistent, and in some episodes, he is completely naked. This means that the two names the basic design was referred to could indeed be separate people.
He is a light blue fish with a dark blue dorsal fin, on the sides of his head, there are light red-purple spots. He has light salmon lips and black eyes. He wears a white shirt and a red speedo.
He appears to be ill-tempered and cynical in most of his appearances. His name was revealed in the episode "Roller Cowards." In "Porous Pockets," Bert calls him "Bill." However, his name was confirmed in "Gary in Love" as Harold. He has been voiced by both Doug Lawrence and Dee Bradley Baker. Lawrence's portrayal gives him a more common American accent, while Baker's gives him an English one.
"Hey, funny guy! I got a joke for you! What smells rotten and puts people to sleep?"
SpongeBob: "Ummmmm... noxious gas?"
Harold: "No, your act!"
"Why, yes it is." (first line, to SpongeBob)
"You! Come back here! You've gotta help me clean this place up!"
"You brought this on yourself, Harold. I did not Harold! Yes, you did Harold. Harold, I did not!"
"Big meaty claws!"
"Bring it on old man, bring it on!"
"Oh no! Chum!"
"That thing's green! Ah ha ha ha! Green!"
" Aw, does baby want a Krabby Patty, hmm? "
[Covering his mouth] "Ooh! Oh..." [Faints]
"How dare you! You toyed with our emotions!" [Pointing at Plankton. Pokes him]
"So, you like kicking butts, do ya? Well we'll show you old man!"
"Your hideous white skin just ruined the party! I mean look at you, do you hug your mother with that skin?" [When SpongeBob's caramel skin smashed]
In "Roller Cowards," he was called Harold while in "Porous Pockets," he was named Bill. This may be because his name was changed to Harold for the "Fiery Fist O' Pain" Commercial. Though in "Gary in Love," he talks to himself, calling himself Harold. It can be presumed that his real name is Harold.