Tidy

Cute Little Fangs: Tidy has prominent canine teeth compared to the rest of the cast.
The Ditz: Tidy isn't the brightest bulb thrown into the ocean. Her reaction to being given a bag with a shirt in it was to wear the bag like a shirt.
Hammerspace Hair: Played with. She's got so much rubbish in her ponytail that it actually gives it a lot of volume, which goes away once the junk is removed.
Innocent Fanservice Girl: Except wearing a plastic bag as a tanktop, Tidy is practically stark-naked. Not only she doesn't even begin to feel any hint of embarassment nor does she grasp any concept of sexuality, due to her naivety and giddyness it's nigh-impossible to view her in any lewd way.
Ironic Name: Ricky and Spencer bust a gut laughing when they learn that the mermaid who lives her life surrounded by filth and rubbish is named "Tidy".
Plucky Girl: Cheerful and full of energy, she finds wonder and excitement in the trash she ends up surrounded by.
Red Oni, Blue Oni: The red to Neptaan's blue. Tidy is cheerful and energetic, while Neptaan is more timid and melancholic.
Shipper on Deck: Tidy tries to act as a wingmaid for Ricky and Spencer, much to the former's embarrassment.
Trademark Favorite Food: Tomatoes, ever since Ricky introduced them to her. She also seems to be fond of cigarette butts.
Training Montage: She undergoes one in a strip in order to get strong enough to open a pickle jar.

Ricky

Cannot Spit It Out: He has a crush on Spencer, but he becomes flustered and panics whenever it comes up.
Dreary Half-Lidded Eyes: His eyes are often drawn this way, alluding to his aloof nature.
Mr. Vice Guy: He loves money, and has a mischievous and short-tempered streak that gives him a rougher edge than Spencer, but he is just as caring, often going out of his way to help Tidy.
No Celebrities Were Harmed: The One Million Subscribers Q&A reveals a major inspiration for his design is Brendon Urie.
Red Oni, Blue Oni: The red to Spencer's blue, and it's reflected in the colors they wear. Ricky tends to have a quicker temper, particularly towards Tidy’s cluelessness of human life, while Spencer remains calm and keeps Ricky in check.
Shout-Out Theme Naming: He was given the name "Ricky" to parody Prince Eric from The Little Mermaid.

Nice Guy: Polite and mild-mannered, he's always kind to everyone and warmed up to Tidy immediately.
Red Oni, Blue Oni: The Blue to Ricky's red, and it's reflected in the colors they wear. Ricky tends to have a quicker temper, particularly towards Tidy’s cluelessness of human life, while Spencer remains calm and keeps Ricky in check.
Stage Magician: He has an interest in magic tricks and is able to perform a few.

Dreary Half-Lidded Eyes: They add to his lethargic and melancholic appearance.
Moreteeth Than The Osmond Family: He has a mouth full of shark-teeth which contrasts his general gentle melancholy initial appearance.
Red Oni, Blue Oni: The blue to Tidy's red. Trashboy is timid and melancholic, while Tidy is more cheerful and energetic.
Religious and Mythological Theme Naming: "Neptaan" is a corruption of Neptune, the Roman god of the sea.
Scars Are Ugly: After his hose-clamp is removed, he feels ashamed of the scars left on his neck, until Ricky and Spencer give him a bowtie. Later on, they accidentally cut the bowtie while freeing him from twine and freaks out again, until Tidy, Ricky and Spencer show him that they also have scars and manage to make him stop being self-conscious about it.
Shrinking Violet: Shy and timid, he was initially reluctant to meet Ricky and Spencer out of fear.
Significant Anagram: Ricky renames him by taking the letters of "Neptaan" and rearranging them (as well as turning the p upside down) so when combined with Tidy's spelling out of her name, they make "neat and tidy".
Theme Naming: Ricky invokes this by renaming him "Neat", so like Tidy, he ironically has a cleanliness-themed name.

An Arm and a Leg: A fin actually. Trashdad's fin gets tangled, and nothing Tidy does can untangle him. He ultimately has it amputated.
Eyes Always Shut: Has never been seen with his eyes open.
Gentle Giant: Very large, but also incredibly sweet-natured and laid-back despite his predicament.
Red Eyes, Take Warning: Subverted. The very first thing we see when meeting him is a huge, menacing silhouette with glowing red eyes. He's about the sweetest person one would ever meet.
Vague Age: While the One Million Subscribers Q&A gives out Tidy, Ricky, Spencer and Neat's ages in terms of specific numbers, (12, 17, 17 and 14), Trashdad's age is listed as "very old".

An Arm and a Leg: Slot is missing his right eye and his right arm.
Bait-and-Switch: Their debut made it seem at first they were going to attack Trashdad after he loses his tailfin, but it turns out they're his loyal followers. Later, it seems like they're about to viciously attack humans, but instead, they just steal their belongings and present them to Trashdad. In one case, these stolen items were presented in a bag shaped like a tied-up body was inside.
The Big Guy: Torx plays this role, due to having a larger build than the others and enough strength to break part of the dock.
Extreme Omnivore: When they offer a depressed Tidy some food, they're shown eating part of their non-food offerings. Hex, an old shoe. Square, a plate of cigarette butts. Torx, a book. Slot, a tin can.
Law of Chromatic Superiority: Hex is the leader of the squad, and he wears red.
Pyromaniac: They have an obsession with fire, to the point they cut their tailfins so it looks like flames. They stash a lot of explosives and flammable objects in their lair, and have used them to destroy the dock.
Revenge: They have it out for humans because of their pollution costing Trashdad his tailfin.
Symbol Motif Clothing: All four of them have a picture of fire on their clothes.
Theme Naming: They're named after the screws they wear on their left ear.
Trademark Favorite Food: Slot loves tomatoes so much, he pretends to hate them at first so he wouldn't have to share with the others.