The following characters has appeared in every episodes in the series, also having major roles in each episode.
The main protagonist and is described as a "party guy". Milo is a cute 16-year-old fish loves going on wild adventures with his friends. He will do whatever it takes to save the world from boredom, even if it takes him to crazy situations. Milo is also portrayed as lazy, and enjoys eating apples. His brother is a catfish named Oscar and his friend is a goldfish named Bea. Voiced by Kyle Massey.
Appearance: Short, slender, aqua skin with blue-green polka dots, blue-white undertones, round cheeks, orange lips, blue mohawk hair, blue striped bodysuit, dark green flippers and fins, blue dimples, usually seen with a smiling face
The detourtagonist and the nervous and neuteral brother of cute and sweet Milo. He is nervous and has a big crush on Bea, one of his friends. Oscar likes playing fantasy games, video games, and other nerdy activities with his friends. He only joins Milo's crazy adventures cause Milo makes him join. Oscar is currently dating Angela, who, like him, enjoys video games. Voiced by Justin Roiland.
The tritagonist and the friend of Milo and Oscar. She wants to become a famous actress and is overly-dramatic about everything. Oscar has a crush on her, but she doesn't notice. Bea also has embarrassing parents who constantly humiliates her, but she still loves and cares for them. Voiced by Chelsea Kane.
Freshwater High students
These characters are students of Freshwater High. Most of them appear in the majority of the episodes throughout the series.
One of Bea's best friends and the secondary tritagonist of the series. Shellsea is independant and focused, and loves to pick out who is less attractive than her on the yearbook. She also loves makeup, dating, and boys, especially Steve Jackson. Shellsea usually gives Bea very helpful advice when she needs help. Voiced by Kari Wahlgren.
Albert is a very smart and nice fish in Freshwater High and is one of the secondary tritagonists. Albert is brainy and is described as nerdy, which, along with his lack of physical strength, is the reason why Jocktopus bullies him and Albert's best friend, Jumbo Shrimp. Voiced by Atticus Shaffer.
Albert's best friend, who, like Albert, enjoys studying and lacks social skills. Jocktopus often bullies both Albert and Jumbo due to their nerdy behavior and lack of physical strength. Jumbo enjoys fantasy role-playing games, which he plays with Albert, Oscar and Milo. Voiced by Steven Christopher Parker.
A crazy clam who goes to Freshwater High. Clamantha enjoys cheerleading and has losts of spirit. Clamantha has an insane crush on Oscar, to the point of her being completely obsessed with him. Voiced by Alex Hirsch.
|Jocktopus Ludwig Vandenbush||
The main antagonist and the school bully. He's an octopus who loves beating up other people. Jocktopus plays in the football team, has his own gang to help him with his bullying, and has a girlfriend named Piranhica. Voiced by John DiMaggio.
Appearance: Slightly obese, goldenrod skin, burgundy hair, scars on his eyes, orange lips, apricot undertones, purple shirt with an "8", baseball cap
A small fish who is another one of Bea's friends. Finberley has an extremeley notable crush on Steve Jackson, who she adores. Finberley is also prone to having bad luck and getting injured, which serves as a running gag in the series. Voiced by Kimberley Mooney.
Personality: Kind, sweet, friendly, cute, anxious
Appearance: Slender, red hair, yellow skin, freckles, salmon lips, braces on her teeth, magenta shirt with pink polka dots
One of Bea's friends. Esmargot is a strange, slimy, sticky green sea slug who leaves a trail of slime behind wherever she walks. She feels sick all the time, and does not really have a bright spot, but is still friendly. Voiced by Rachel Dratch
Personality: Friendly, kind, sweet, inquisitive, curious, confident
Appearance: Slender, green skin with an inchworm belly and olive green spots, pink lips, purple hair with pigtails, pink shirt with white collar and two buttons,
One of the bigger fish in Freshwater High. Koi is one of Bea's friend and although she is big, she is very gentle and has a huge heart. Koi can only say "Wuh!" in a gruff, deep voice, which other characters seems to understand. Due to her strength and height, Koi is in the schools basketball team. Voiced by Rachel Dratch.
Personality: Mute, clumsy, sweet, perky, flirty, annoying
Appearance: Slender, peach skin, rosy cheeks, red-orange lips, red hair with tropical tattoos, red flippers and fins
A selfish crab and the primary antagonist who attends Freshwater. He is very rich and has a crush on Bea, which angers Oscar. He also seems to treathen people to do what he says. He is somewhat of a bully to Milo. Voiced by Josh Sussman.
Personality: Greedy, slender, snobby, sarcastic, mischeivous, rude, tragic, mean
Appearance: Small and slender, peach skin, apricot lips, brown hair, little right claw, big left claw, scuttling feet
|Bo Gregory||A student who is known for having all sorts of tractors and trucks, all of which is from his farm. Bo Gregory even has a farm in his locker in Freshwater High. Voiced by Maxwell Atoms.|
|Steve Jackson||The most popular fish in Freshwater High. Steve attracts girls of all sorts, and is aware of his popularity. Steve is a very nice person, however. Also, playing sports and dating girls is what he loves to do. Voiced by Greg Cipes.|
|Dan and Ann Chovie||The Chovie twins are the most childish and immature fish in Freshwater High. They may seem weird and odd, but they both have lots of school spirit. Voiced by Jane Carr.|
|Hugh Edmondson||A foreign exchange student from France, Hugh Edmondson speaks with a Frecnh accent, and he has a very elegant and rich house. Hugh isn't a man with a lot of words, though. Voiced by Maxwell Atoms.|
|Goth Fish||Lead by the gloomy Razor V. Doom, the goth fish are the most monotonous and boring fish in the school. They are usually in the edge of the hallways by themselves, only getting along with each other and not with other students. The members include Scabbo, Goth Squid, and Bleak Molly.|
|Piranhica||Jocktopus's cruel girlfriend. Piranhica loves sweet talking him in public, which annoys Jocktopus sometimes. Unlike Jocktopus, Piranhica is intelligent and is very strong, despite her small appearance. Just like Jocktopus, Piranhica loves provoking people and watching nerds get beat up. Voiced by Laura Ortiz.|
|Pass, Punt, and Fumble||Pass, Punt, and Fumble aren't the brightest ones in school, but this trio spends most of their time teaming up with Jocktopus to make student's life miserable by bullying them as much as possible. Voiced by Dave Wittenberg, Alex Hirsch, and Roger Craig Smith.|
|Student Council||Three lobsters who organizes important events for the school. They are all sophisticated and smart, and all three of them speaks in the same exact monotonous voice as the other. Two of them are boys, and one of them is a girl. Voiced by Derek Evanick.|
|Lonnie||The marching band leader of Freshwater High. He may seem like a regular band geek, but Lonnie and his marching band are very tough, and they could get rid of anyone's instruments, even if they have to fight for it. Voiced by Doug Brochu.|
|James Sardine||As the smallest fish in Freshwater High, James Sardine is more of a silent type, but he can still date girls, and driving his own car into school doesn't get in his way.|
|Freshwater High cheerleaders||The Freshwater High cheer squad are a group of cheerleaders who loves cheering as much as dating. They are all nameless until 2012 to 2014, and they got taken away by Bud from the tanks. For unknown reasons, they come back in future episodes.|
|Hoodie||Hoodie is a fish who always wears a blue sweater with a hood over his head, hence his name. Not much is known about him since he's one of the background characters in the series. Voiced by Kevin Michael Richardson.|
|Dumpy Muffins||Another background character in the series, Dumpy is one of the hall monitors in Freshwater High. Dumpy is quiet and was nameless until the Season 2 episode "Milo on the Lam". Voiced by Noah Z. Jones.|
Another background character in the series. a 17-year-old goldfish. Voiced by Kari Wahlgren (2011-2014) and Chelsea Staub (2011).
|Kevin||Nobody knows who this guy is. He appears in the halls and other areas of Freshwater, and has been left out ever since he got held back in school. He has brown hair, freckles, and orange lips. Voiced by Jason Earles.|
Freshwater High teachers and staff
These following characters are teachers of Freshwater High.
|Mr. Baldwin||Milo, Oscar, and Bea's boring homeroom teacher. Mr. Baldwin is a pregnant seahorse, who seems depressed about his life all the time. Something happened to him in the 1980's, but no one knows what happened. He also loves teaching his children. Voiced by Dana Snyder.|
|Coach Salmons||The gym teacher of Freshwater. Coach Salmons is kind and always wants to do what's best for his students. He enjoys singing, exercizing, and comforting his students. Voiced by Richard Simmons.|
|Mr. Mussels||The drama teacher in Freshwater High. Mr. Mussels thinks everyone has a dream, and that someday, they will rise up and shine. Mr. Mussels speaks very loud, and tries his best to encourage his students. Voiced by Tiny Lister.|
|Ms. Lips||Freshwater High's strange school couselor. She teaches students advice on how to live their life, but it seems that she needs the most help. Because of her long and tall size, she is known for crashing the top of the door with her head. Voiced by Jennifer Coolidge.|
|Principal Stickler||The principal of Freshwater High. Principal Stickler yells orders to his assistant, Nurse Fishington. Stickler takes his job very seriously, and he hates whiny students who gets in his way. Voiced by Jerry Stiller.|
|Nurse Fishington||Nurse Fishington is Principal Stickler's assistant and is also Freshwater High's nurse. She always seems to do Stickler's chores, such as pushing his chair all the time, and Stickler always yells orders to her.|
|Headphone Joe||Freshwater High's janitor, who cleans tanks by sucking the glass. Headphone Joe is laid-back, nice, and could give advises, although he doesn't help much. Voiced by John Caparulo.|
|Dr. Frog||Freshwater High's journalism teacher. He once teached woodshop because everyone hated his journalism course. Dr. Frog now seems to teach many different subjects, such as French class or Science. Voiced by Kevin McDonald.|
|Lunch Lady||The Lunch Lady serves the students food in the lunch hall. She seems to always have a bored expression, suggesting the fact that she doesn't like her job that much. Voiced by Kari Wahlgren.|
|Librarian||The Librarian is an anglerfish who is in charge of the Freshwater High library. He seems to dislike it when students make noises or laugh in the library. He's also seen in Bea's Birthday Surprise.|
These characters are neither Freshwater teachers or students. Some live inside the tank, while others live outside.
|Angela||Oscar's ex-girlfriend. Angela seems to like all of the stuff Oscar likes, such as video games and fantasy role-playing activities. Voiced by Felicia Day.|
|Chief||The boss of the Hokey Poke. Chief runs the restaurant with an iron fist, and doesn't have time for any fun and games. Chief has also left an interesting life behind his current job, such as capturing "Big Blue". Voiced by Blake Clark.|
|Bassy||The waitress at the Hokey Poke. She works alongside with the Chief, so the two of them makes sure the Hokey Poke gets a lot of customers. Bassy takes the orders, while Chief prepares the food. Voiced by Tress MacNeille.|
|Mrs. Goldfishberg||Bea's overprotective mother. She treats Bea like a baby throughout the series, but is friendly and kind-hearted. She once chaperoned for the school dance. Voiced by Edie McClurg.|
|Norman Goldfishberg||Bea's father. Like the mother, he embarresses Bea a lot. He got Bea's mom to marry him by his dance moves. He once chaperoned for the school dance. He has brown hair and light blue lips. Voiced by Noah Z. Jones.|
|Pamela Hamster||Milo's hamster girlfriend who lives in Hamsterwood. She once went to Freshwater to visit Milo, where she suprisingly reveals that she is a gerbil. Pamela broke up with Milo, saying they need a break, and the long-distant relationship won't work out that well. Voiced by Sabrina Bryan.|
|Geckos||Freshwater High's biggest rival. They all live in the Gecko tank, where they compete Freshwater in sports. However, they never won a single game. Voiced by Dave Wittenberg.|
|Bud||The real owner of Buds Pet Shop. Bud is a huge guy (at least to the fish). He mostly spends his time cleaning the floor or doing awkward stuff. Voiced by Dana Snyder.|
|Wilford||A giant cat that roams around the pet store, hoping that a fish could come out. Wilford once chases Milo, Bea, and Oscar through the store.|
|Mr. Nibbles||Pamela's boss at the Hamster studios at Hamsterwood. Mr. Nibbles seems to hate the smell of fish, and complains about the smell whenever he gets near one. Voiced by Jason Alexander.|