The Main Trio
- Voiced by: Kyle MasseySpecies: Siamese fighting fish
A fish who's always looking for some fun. He's usually the class clown, especially in Mr. Baldwin's class. He's always getting his two best friends in many zany adventures, whether they want him to or not.
- Attention Deficit... Ooh, Shiny!: "Fail Fish" implies that he has some sort of attention disability.
- Book Dumb: He is not a good student and he has trouble learning. But one episode showed that he is able to learn new things when he is in danger.
- Blue Is Heroic: Hes light blue with darker blue fins.
- Catchphrase: "Hot dog!"
- Class Clown: In Mr. Baldwin's class.
- Cloudcuckoolander: He's very eccentric and goofy, and is prone to coming up with crazy ideas out of the blue.
- Comedic Underwear Exposure: He is sometimes seen wearing Oscar's underwear for comedic affect.
- The Ditz: He isn't very wise.
- Fat Best Friend: To Oscar and Bea.
- Fat Idiot: A bumbling, obese fish.
- Foil: To Oscar.
- Fun Personified: Always looking for fun, regardless of the situation.
- Heterosexual Life-Partners: With Oscar, of the brotherly, family variety.
- Ignorant of Their Own Ignorance: He's a stupid goofball and usually the reason for episode conflicts, but actually means well nevertheless.
- Kindhearted Simpleton: His kindness and well-meaning intentions make up for his stupidity.
- Large Ham: He lacks an indoor voice and is prone to overdramatic emphasis.
- Life of the Party: Is usually one to party crazier than anyone else.
- Love Interest: Finberley seems to have a crush on him, seeing as how she once requested to go on a lunch date with him and thinks it's cute when Milo gets all weak.
- Nice Guy: While he isn't very wise, he has a quite friendly and endearing attitude.
- Nice Job Breaking It, Hero!: He is usually the cause of conflict.
- No Indoor Voice: Being quiet and him are not on the best of terms.
- Obsessed with Food: He once got hungry in class because Mr. Baldwin's nose apparently looks like a sea cucumber.
- Odd Name Out: Out of the trio, he's the only whose name is not a common one.
- Platonic Life-Partners: With Bea, being one of his best friends.
- Potty Emergency: "Bea Saves a Tree" reveals that he pees in the water rather than the bathroom.
- Trademark Favorite Food: Apples.
- Wacky Guy: He can go overboard with his silliness sometimes.
- Voiced by: Chelsea KaneSpecies: Goldfish
The goldfish who lives across the street from Milo and Oscar. She has a strong desire to become an actress superstar, and often plays the lead in her school plays. When she doesn't act as the voice of reason, she can be a control freak.
- Ambiguously Jewish: Her last name, "Goldfishberg", is a pun on the surname "Goldberg", which is a stereotypical Jewish name.
- Birthday Episode: "Bea's Surprise Party".
- Drama Queen: Justified as she's the star pupil of the school's drama department.
- This is also her main flaw, she can get a little to passionate about even the smallest role.
- Determinator: When she sets her mind to something, Bea just won't stop.
- First Girl Wins: Shes been hanging out with Oscar since the start of the show, and they become a couple by series end.
- Gamer Chick: She'll occasionally join Oscar and Milo in playing video games.
- Girly Girl with a Tomboy Streak: Despite "Sleepover Party" showcasing her as a stereotypical teenage girl, she still has some masculine interests like football and playing video games with Milo and Oscar.
- Green-Eyed Redhead: She has red hair and, as shown in "Oscar is a Playa", she has green eyes.
- Jerk with a Heart of Gold: Sure, Bea is selfish, desperate to reach her dreams, and suffers from Small Name, Big Ego, but at the end of the day, she actually cares more about her family and friends.
- Large Ham: Justified, since she is an aspiring actress.
- Platonic Life-Partners: With Oscar and Milo until she gets together with the former in the Series Finale.
- Plucky Girl: Bea is cheerful, enthusiastic and rarely gives up when she wants something.
- Pride: She has shown signs of arrogance, such as frequently complimenting herself and thinking that she's too talented to be punched, as "Happy Birthday, Jocktopus" shows.
- Redhead In Green: When she dresses as Oscar in "16 Clamandles".
- Single Girl Seeks Most Popular Guy: The object of her affection in some episodes is the local Pretty Boy heartthrob.
- Single Woman Seeks Good Man: During graduation, she realizes she likes Oscar.
- Small Name, Big Ego: Due to her dreams of becoming famous, she often has a high opinion of herself.
- Unwitting Pawn: To Fishington in "Principal Bea".
- Youthful Freckles: Although not that prominent, she has three feckles on her right cheek.
- Voiced by: Justin RoilandSpecies: Catfish
A nerdy fish who likes to play video games and roleplay. When Bea isn't the voice of reason, he usually fulfills this role. His brother's antics may make him cranky at times, but he's always got his back.
- Adorkable: He's socially awkward, has braces, is obsessed with video games, and gets flustered around Bea.
- All Work vs. All Play: The All Work to Shellsea and Milo's All Play.
- Ambiguous Disorder: He's really smart, can't understand, or act, in some social interactions, and in one episode, can't seem to understand jokes or visual gags, so Milo teaches him how to laugh.
- Berserk Button: Just don't let him catch you even touching Bea if you're a jerkass, because he'll be very angry.
- Butt-Monkey: His worries are often a result of his misfortunes.
- Dogged Nice Guy: To Bea.
- Foil: To Milo.
- Meaningful Name: An Oscar is a popular species of aquarium fish.
- Nice Guy: Very nice most of the time.
- Nervous Wreck: And how. He loses his cool over just about everything.
- No Sense of Humor: The "Funny Fish" deals with this.
- Not So Different: When he finds out that Clamantha has an eerie Stalker Shrine of him, Oscar didn't mind too much because he has the same for Bea.
- Pac-Man Fever: His video games usually look very old-fashioned.
- Perky Goth: In the episode where he became one, although he was back to his normal self at the end.
- Pubescent Braces: He wears braces.
- Stalker with a Crush: To Bea, but he is not as bad as Clamantha.
- Voiced by: Josh SussmanSpecies: Crab
A narcissistic crab who's extremely wealthy.
- Asshole Victim: He really does deserve all the bad things that happen to him.
- Author Avatar: He strongly resembles his voice actor, Josh Sussman.
- Catchphrase: "I'm Randy Pincherson!"
- He even says it while he's disguised as a Grandma.
- The Dragon: He'll sometimes assist Jocktopus when it comes to sabotage.
- Jerkass: He's an obnoxious, selfish, Rich Bastard who boasts constantly about his wealth and shows little respect to his fellow classmates.
- Laser-Guided Karma: He'll usually get what's coming to him in most episodes.
- Narcissist: He doesn't care about anyone but himself.
- Penny-Pinching Crab: He is a crab student know chiefly for two things: being very wealthy and being very narcissistic about his wealth. In the episode "Dollars and Fish", he plays the part of the strict, unforgiving loan shark whom Milo gets his money from... and who has no problem pressuring defaulting debtors with his claws.
- Rich Bastard: He's very wealthy and acts quite boastful about it.
- Voiced by: John DiMaggioSpecies: Octopus
An octopus who plays on the school football team. Known to be very aggressive to his fellow classmates and is best friends with Albert Glass. He has a soft side that he rarely shows to anyone.
- Birthday Episode: "Happy Birthfish, Jocktopus".
- The Bully: Of Freshwater High.
- Dumb Jock: He's a complete meathead.
- A Day in the Limelight: "Joctopizza", which has really minimal involvement from the main trio.
- Gonk: He is hands down the ugliest character on the show, and is often regarded as such in-universe.
- Heterosexual Life-Partners: With Albert Glass.
- Hidden Depths: According to his Fishbook profile, he likes doing chores around the house a bit.
- Hulk Speak: He speaks in this tone of voice.
- Jerkass: He bullies his fellow schoolmates for no apparent reason.
- Jerk Jock: Is very aggressive and disrespectful to his fellow classmates.
- Jerk with a Heart of Gold: On rare occasions, he has moments of being nice.
- Large Ham: As a result of his aggressiveness and Hulk Speech.
- Non-Standard Character Design: He's obviously taller and bigger than the other fish characters.
- Odd Couple: With Piranhica.
- Official Couple: With Piranhica.
- Odd Friendship: With Albert Glass.
- Punny Name: His name is a play on the words "jock" and "octopus".
- Third-Person Person: He always refers to himself in third person.
- Vitriolic Best Buds: With Albert Glass, as he ruthlessly torments and bullies him.
- With Friends Like These...: He claims to be friends with Albert, yet the latter is frequently on the receiving end of his bullying and torment.
- Voiced by: Laura OrtizSpecies: Piranha
A piranha who is Jocktopus' girlfriend. Her short temper makes her less pleasant than her boyfriend, and that's saying something!
- Voiced by: Kimberley Mooney
A clumsy fish who can be very emotional and hyperactive at times. She always tries her best to keep a smile on her face.
- Adorkable: She sports braces, is quite friendly, and gets hyper over her opportunities.
- Butt-Monkey: She has gotten this status since Season 2.
- Cute Clumsy Girl: She sure is cute, yet clumsy.
- The Cutie: She gets very emotional at times, often leading to cute, Genki Girl momments of Squee!.
- Genki Girl: She's one cute and hyperactive fish.
- Girlish Pigtails: Has these.
- Nice Girl: Has a sweet and endearing personality.
- Older Than They Look: She is a teenager but looks like a little girl. Her human counterpart in " Pool Party" further displays this.
- Pubescent Braces: She's a teenager and sports braces that, along with her pigtails, make her resemble the "awkward youth" state.
- Running Gag: Getting hurt unintentionally.
- Voiced by: Kari WahlgrenSpecies: Jewelfish
A jewelfish who's into fashion and being cool. She's Bea's closest friend outside of Milo and Oscar, and is seen occasionally giving advice to her. She loves to gossip and eat.
- All Work vs. All Play: The All Play to Oscar's All Work. (One of Two)
- Beauty Is Bad: She may be attractive, but she's an Alpha Bitch. However it's downplayed in that she's more of a Lovable Alpha Bitch (see below).
- Big Beautiful Woman: As a human in "Pool Party Panic".
- Lovable Alpha Bitch: She is popular and sometimes looks down on Oscar and Milo, but is generally a good friend to Bea.
- Phoneaholic Teenager: She loves chatting on the phone.
- Pink Means Feminine: Shes pink all over.
- Valley Girl: She is a stereotypical teenage girl who uses words like "Bae" and "Girl" often.
- Voiced by: Alex HirschSpecies: Clam
A kooky clam who serves as captain of the cheerleading team and the editor of the school yearbook. She has the hots for Oscar and isn't afraid to hide it, even when he's taken.
- Ambiguously Human: Compared to the rest of the humans in "Pool Party Panic", her human self looks absurd.
- Cloudcuckoolander: Being a clam among her mostly-fish students and all, she's very strange...
- Out of Focus: Has far less screentime in the second and third seasons, due to her voice actor going on to create another show.
- Stalker with a Crush: Towards Oscar.
- Voiced by: Rachel DratchSpecies: Slug
An awkward slug who's usually the pessimistic one in the group, and Albert Glass' girlfriend.
- Voiced by: Rachel Dratch
A big Koi fish who can only speak in "wuh"s. Compared to her friends, she's surprisingly the most gentle one.
- A Day in the Limelight: "Koi Story".
- Gonk: Her appearance is quite distorted.
- Gonky Femme: She is big and gonky, has a deep voice, and is a Dumb Muscle, but she acts as girly as the rest of her friends.
- Huge Schoolgirl: Serves as the giant one among her little fish classmates.
- Intelligible Unintelligible: Despite having a limited vocabulary, everyone understands what she's saying.
- The Unintelligible: She usually only speaks in limited gibberish.
- Voiced by: Lil' Twist (Season 3)
A famous pop star with tons of fans. His charming ways bring many fangirls to his side.
- Ascended Extra: Originally started out as just a singer who had no lines outside of singing. He got larger parts as the show went on.
- Your Cheating Heart: After leading Bea on during Camp Camp, she over hears her seducing another girl the same way he did to her.
- Expy: He's an in-universe stand-in for Justin Bieber.
- Voiced by: Felicia DaySpecies: Angelfish
An angelfish who's the captain of the rivaling Saltwater High Debate Team. She can be introverted at times and is heavy into gaming.
- Voiced by: Atticus ChafferSpecies: Glass fish
A glass fish who is one of the smartest in his class. He skipped several grades, making him the youngest of his class.
- Adorkable: He is a friendly, innocent little nerd.
- Butt-Monkey: Jocktopus often bullies him.
- Child Prodigy: Implied. He skipped a few grades and he sounds really young compared to the other students on Freshwater High.
- Nice Guy: Albert is usually shown to be a pretty friendly guy.
- Vocal Evolution: His voice has gotten deeper near the end of the series due to his voice actor experiencing puberty.
- Voiced by: Steven Christopher ParkerSpecies: Shrimp
A giant shrimp who's a pretty big nerd. He's done many science experiments and is very good at robotics and computers.
- Voiced by: Greg CipesSpecies: Red salmon
The most popular boy at Freshwater High School.
- Voiced by: Dana Snyder
A pregnant seahorse that teaches at Freshwater High. He's had trouble with his social life since 1986, at least until Milo, Oscar, and Bea came along...
- Apathetic Teacher: Due to the idiocy of his students.
- Deadpan Snarker: If his students provoke him with his idiocy, he'll make sarcastic remarks behind their backs.
- Mr. Seahorse: He's literally a seahorse and was once pregnant in his life.
- No Name Given: His first name has never been revealed.
- The Slacker: He is sometimes seen sleeping on the job.
- Voiced by: Jennifer Coolidge
A squid who's the school guidance counselor that has some issues of her own. She can be quite absent-minded at times. She has a catfish named Attila that she adores very much.
- Voiced by: Richard SimmonsSpecies: Salmon
The P.E. teacher who encourages having a healthy lifestyle.
- Dark and Troubled Past: It was implied that he's fought in war before, although it was only brought up once.
- Voiced by: Vanessa Marshall (Mouse), Kari Wahlgren (Snake)
A pair consisting of a mouse and a snake living together on a terrarium in front of the fish tanks. They usually watch what's happening in them.
- Credits Gag: Their commentary usually serves as one, though sometimes it can be unrelated to the episodes plot, or none at all for a handful of episodes.
- A special one happens at the end of "Pool Party Panic", in which they turn into humans, then pigs and then vegetables before they turn back into their animal selves.
- Odd Friendship: Well, duh. Snakes eat mice.
- Voiced by: Dana Snyder
The owner of the pet shop the series takes place in. He usually has some antics of his own at the start of the program.
- Non-Standard Character Design: He's photorealistic like the rest of the world, instead of cartoony like the characters.
- Once an Episode: He appears at the beginning of each episode, doing something either related or unrelated to the episode. If he appears anymore afterwards depends.
- Red Baron: He's called "The Big Blue" by Hokey Poke's chief. And seeing what happened between them as shown in "Peopleing", he has a VERY good reason to call him like that.
- The Dreaded: To Chief.
- I Ate WHAT?!: While he doesn't seem to realize it, he ate Bea and Milo's apple experiment in "Fuddy Duddy Study Buddy" once it fell into a trashcan.