Captain K'nuckles is not very active and prefers to spend his time in Bubbie's mouth or eating candy at the Candy Barrel (despite him and Flapjack not having enough money or any money at all, and will risk a lot to get 'free eats'). He usually likes to tell stories of his many adventures to Flapjack and, because of this, the young boy fails to see K'nuckles as the inactive good-for-nothing that he is.
K'nuckles is not very bright, as shown in his attempt to do his laundry. Instead of going to the laundromat, he throws his clothes into the ocean and expects them to be thrown back into his arms once they're clean. He also shows some disgust towards emotional moments, but in the episode K'nuckles and His Hilarious Problem it is revealed that after he had too much candy he went into some sort of inebriated state where he confessed to Flapjack that he was the only friend he ever had and that he also cared for Bubbie. After his confession, he began to cry and soon passed out.
Though K'nuckles is shown to neglect Flapjack and their friendship on multiple occasions, he also shows some compassion toward Flapjack and is clearly fond of him as a friend. He might even look at Flapjack as the son he never had. K'nuckles seems to have an obsession for Maple Syrup which he also refers to as his friend. He is also illiterate, as seen in many episodes, particularly in "Candy Casanova" where he writes a letter to Candy Wife (which Flapjack reads) using only pictures; in Wishing Not So Well, he creates a to-do list using pictures as well.
Though his appearance may confuse some, K'nuckles considers himself Human as shown in the episode "Oh You Animal!". Captain K'nuckles has a severe case of "whiskers" (which causes his entire head to appear blue) as well as numerous tattoos and some scars as evidenced when he is seen shirtless (which is quite often). The whiskers surrounding his mouth are ironically fake, of which he is greatly ashamed, as seen in "Beard Buddies". He is characterized by his long, orange nose, that he is very fond of. He does not have any natural limbs left; they have all been replaced by crude, prosthetic wood including the legs and hands. Even his rear end is wooden, being a just a plank attached with a belt, although he eventually reclaimed his real bottom (only to lose it in the same episode, Sittin' Muscle). He has an odd waddle when he walks because of this, and makes squeaking noises when he moves. K'nuckles lost one of his hands to a food processor at a young age.
He carved himself a new hand out of wood, but unfortunately, it was a mirror image of the one he needed. He then amputated his working hand and carved a second wooden hand. Fortunately, both prosthetic hands are in excellent working order, allowing K'nuckles to do anything a person with real hands can do (it has been said that he based them off the designs of the best-created mechanical hands ever created, although it is unknown if this is real or he just said this to boost his ego).
K'nuckles constantly wears a red pirate hat and jacket over light brown striped shirt underneath. He also has also been shown to wear bloomers, as "Real men don't wear pants." (That's a Wrap) When his irises are rarely seen, it is shown that he has light brown eyes. He also has black circles around his eyes. He enjoys talking to birds. In the episode "Gone Wishin'," it has shown K'nuckles returning to his original "normal" human form after making a wish from a mermaid's heart. It is unclear, however, if this is truly his actual normal form.
- K'nuckles's voice actor is Brian Doyle-Murray in the series, only in Cartoon Network: Punch Time Explosion, was he voiced by Nolan North.
- K'nuckles occasionally goes out to 'business', where he apparently talks to gulls which he thinks are his parents.
- According to the episode "K'nuckles is a Filthy Rat," K'nuckles' says his parents were turned into gulls by a magical genie, and plans to help them.
- Captain K'nuckles apparently has been Snarked before.
- K'nuckles lost all of his body parts in Oh Brother, Hand It Over, Fancy Pants and Under the Sea Monster.
- K'nuckles has been mistakenly referred to as a pirate on some few occasions. He even referred to himself as one to Lady Nickelbottoms.
- Until Shave And A Haircut, Two Friends, K'nuckles has not removed his hat. However he didn't wear it for most of Off With His Hat.
- Despite many episodes drawing attention to his inability to read, at the end of Revenge, he handed Flapjack a letter that he seemingly wrote. It is possible, but unlikely, he dictated the letter, or that Flapjack somehow interpreted the season two.
- Nobody knows what species he is in Oh You Animal!, the lady for "Stormalong Second Chance for Boys" mistakes him for a strange animal, however in the episode Gone Wishin' it is revealed that K'nuckles was actually a human under some sort of curse as he stated to be wished back to normal.
- K'nuckles has a mood for wrestling, but always fails. This happened in the episodes Kid Nickels, Behind the Curtain and Bam!
- He may be in his 70s, as once when the members of the navy said "You have the body of a 70 year old man" which was confirmed in Bam! K'nuckles said "I'm 11 times bigger than you" to Flapjack, who is 7 years old. This could mean that he is 77 years old.
- K'nuckles is the self-proclaimed Moon King.