The main character. A talking dog.
- Afraid of Needles: Is afraid of shots, but not as much as Skits.
- Bad Liar: Whenever she tells lies, they're not very convincing. She states in "Martha's No Dummy" that it's because she's a dog, and all dogs are bad liars.
- Big Eater: Has limits, but eats a lot.
- Berserk Button: Has several pet peeves, including baths, Nelson (formerly cats in general), squirrels, fleas, "no dogs allowed" signs, the TV shows "Mushy Duck" and "Malcolm the Monkey" and power outages.
- Breaking the Fourth Wall: During the between-episode segments.
- Does Not Like Spam: She doesn't like rhubarb pie, cream or the burgers that Big Burgertorium serve.
- Dogs Hate Squirrels: Finds squirrels annoying.
- Fantastic Racism: Used to have this against cats, possibly due to Nelson, but it went away since meeting Kitten.
- Fear of Thunder: Is afraid of thunder, but tries to deny it.
- Filthy Fun: She likes rolling in mud, trash and dead snakes.
- Hair of Gold, Heart of Gold: Has yellow fur and is kind.
- Hates Baths: She hates baths a lot, which is a plot point in "Martha Makes Scents" and "Martha Takes a Stand" but crops up a lot in other episodes.
- Ingesting Knowledge: When she eats something that contains knowledge (like the alphabet soup), she ends up having that knowledge.
- Like Brother and Sister: With Skits.
- Literal-Minded: Downplayed. Tends to take an expression she hasn't heard of literally.
- Potty Failure: Had this (wetting the rug) as a puppy.
- Power-Up Food: Alphabet soup makes her able to speak.
- The Protagonist
- Really Fond of Sleeping: Martha the dog really likes to take naps.
- Speaks Fluent Animal: Can speak Dog due to being, well, a dog, and she can also understand Cat, but cannot actually speak it. She can also communicate with sheep, birds and squirrels.
- Talking Animal: Can talk, but is still very much a dog.
- Trademark Favorite Food: Meat and alphabet soup.
- Why Did It Have to Be Snakes?: Is afraid of thunder.
Martha's ten-year-old owner.
- Compressed Vice: In "Helen's All Thumbs", she takes up video games but must give them up due to becoming obsessed with them and hallucinating due to them. The video game she played never affected anyone else negatively and Helen has never shown any troubles that would make her that way.
- Does Not Like Spam: She doesn't like rhubarb pie.
- Fleeting Passionate Hobbies: Takes up video games in "Helen's All Thumbs", but has to give up after getting obsessed.
- Friend to All Living Things: Likes dogs, and helped convince Martha that cats aren't all bad.
- Injured Limb Episode: Has a minor sprain on her ankle in "Dog Daze" that's gone by the next day.
- Limited Wardrobe: Mainly wears black flats, hot pink tights, a hot-pink-and-orange striped skirt, and a hot pink jacket. Wears light pink pyjamas to bed.
- Performance Anxiety: Has a bit of this, but not as much as Carolina.
- Pink Girl, Blue Boy: The pink girl to TD's blue boy.
- Red-Headed Heroine: Is one of the good guys and has red hair.
- Skirt over Slacks: Mainly wears a skirt over leggings.
- Shrinking Violet: Downplayed, she's shy, but not as shy as most shrinking violets.
- Why Did It Have to Be Snakes?: She's scared of ghosts.
The Lorraines' other dog.
- Afraid of Needles: He is scared of the vet due to getting shots.
- Big Eater: Again, he has his limits, but still eats a great deal.
- Big, Friendly Dog: Friendly, and bigger than Martha.
- Cloud Cuckoo Lander: Is a bit of a goof.
- Fantastic Racism: Used to have this against cats as a puppy.
- Filthy Fun: Just like Martha, he enjoys rolling in mud and trash, although doesn't mind being bathed.
- Green-Eyed Monster: Got jealous of Martha's ability to talk in the first episode with him in it.
- The Klutz: Due to his speed and size.
- Like Brother and Sister: With Martha.
- Literal-Minded: Same as Martha.
- Why Did It Have to Be Snakes?: Is afraid of hockey pucks ever since chasing one got him in danger. Is also afraid of needles.
- Urine Trouble: Pees on Mariella's rug as a puppy.
- Birthday Episode: Her birthday happened in "Martha Runs the Store".
- Compressed Vice: In "Martha's Dirty Habit", she shows disdain for Danny's between-meal snacking, which she's never done before or since.
- Gratuitous Spanish: Sometimes puts Spanish words into her sentences.
- Green Thumb: Is a gardener.
- Happily Married: To Danny.
- In-Series Nickname: Martha calls her "Mom", even though she's clearly not her mom.
- Invisible Parents: We never find out who her parents are or were, not even when Helen draws her family tree.
- Supreme Chef: Is a pretty good cook and does most of the cooking.
- Compressed Abstinence: Tries to give up snacking in "Martha's Dirty Habit", despite having no reason to (not sick, no eating disorders, not overweight etc). Justified because he was trying to cheer Martha (who was trying to abstain from digging) up.
- Green-Eyed Monster: Was jealous of Mariella and Martha as they were both bilingual, but he wasn't.
- Happily Married: To Mariella.
- In-Series Nickname: "Danny", and Martha calls him "Dad" even though he is clearly not Martha's dad.
Helen's little brother.
- Accidental Misnaming: When he tries to say Martha's name he often misses out the 'h' or the 't'.
- Baby Talk: Cannot talk properly yet.
- Canon Foreigner: In the original books, Helen appears to be an only child.
- The Diaper Change: He gets his diaper changed offscreen several times in the series.
- In-Series Nickname: "Baby Jake" or "Jakey".
Helen's eleven-year-old cousin.
- Fantastic Racism: Not so much anymore, but she used to have this against dogs.
- The Fashionista: Is serious about clothes.
- Gratuitous Spanish: Sometimes has this with Spanish.
- Jerk with a Heart of Gold: Is a little arrogant and bossy and disses peoples' clothing, but has her heart in the right place.
- Large Ham: Can be dramatic when she's sad or mad.
- Missing Mom: Her mother is never seen or mentioned.
- Performance Anxiety: In "Martha's Got Talent".
- Serious Business: Due to being a fashionista, she takes fashion really seriously.
- Attention Deficit... Ooh, Shiny!: Downplayed, but he's a bit easily distracted.
- Cloud Cuckoo Lander: A bit, has a vivid imagination and often thinks up crazy stuff.
- Genius Ditz: Is a bit of a Cloudcuckoolander, but is actually a very smart kid.
- Hair of Gold, Heart of Gold: Has blond hair and is kind.
- Insane Troll Logic: He doesn't mean to troll, but his logic can be pretty weird sometimes.
- Large Ham: Likes to give it all he's got.
- Literal-Minded: Has moments where he takes things literally.
- Nice Guy: Is hardly ever rude.
- Nightmare Fetishist: Likes spooky things, the spookier the better.
- The Pollyanna: Is hardly ever sad.
- Potty Emergency: Once has one during a The More You Know segment about health and again during class in "TD and the Light Bulb of Doom"
- Speaks Fluent Animal: Averted, but he does want to in "T.D. is Talking Dog".
- Cloud Cuckoo Lander: Is a bit of a goofball.
- Gadgeteer Genius: Is an inventor.
- Genius Ditz: Goofy but at the same time highly intelligent.
- Hair of Gold, Heart of Gold: Just like his son.
- Like Father, Like Son: Both he and T.D. have blond hair and are goofy and inventive.
- Mr. Fixit: His job.
- Nice Guy
T.D.'s mother and the vice-principal of the school.
- Friend to All Living Things: Likes animals as much as the other kids.
- Genki Girl: Is female and energetic.
- Hair of Gold, Heart of Gold: Kind, and blonde.
- The Klutz: Is very clumsy.
- Limited Wardrobe: Which consists of glasses, brown pants, red shoes, and a green T-shirt with a purple stripe.
- Lovable Jock: Very sporty, but not a Jerk Jock.
- Sibling Rivalry: With Ronald.
- Tomboy: Compared to Helen.
Alice's teenage brother.
- Big Brother Bully: Takes pleasure out of annoying Alice.
- The Bully: Not only annoys Alice but annoys the other children too.
- Deadpan Snarker: One of his traits which the other characters don't like.
- Gadgeteer Genius: Likes computers and whatnot.
- Jerk with a Heart of Gold: Is a bit of a bully, but actually has a soft spot.
- Pet the Dog: secretly bought a pair of dance shoes for Alice when the latter took an interest in ballet.
- Sibling Rivalry: With Alice.
- The Bully: Bullies Martha and Skits a lot.
- Cats Are Mean: Is a cat and mean. However, he's the only mean cat in the series.
- Never My Fault: Always tries to pin the blame on Martha.
- The Rival: To Martha and Skits.
- Wounded Gazelle Gambit: Pulls one in "Martha Takes the Cake" by making it look like Martha bit him in order to have her be put in the garage, then frames her further by leaving cake crumbs near her.
- Ambiguously Brown
- Black and Nerdy: Is black, and also has an unquenchable thirst for knowledge.
- Book Worm: Reads a lot.
- Green-Eyed Monster: Was jealous of the babies in the episode "Martha Plays Favourites"
- Lovable Coward: Is a bit fearful.
- Nerd: He's nerdy about literature, dinosaurs and Einstein.
- The Smart Guy: Is very smart.
- Why Did It Have to Be Snakes?: He is scared of clowns, tigers, bats, and especially bugs.
Two con men.
The animal shelter owner.
- Ambiguously Brown: Has a lighter brown skin like Carolina's.
- Comically Missing the Point: He tells Martha to bark once for one dog food and twice for the other, forgetting Martha can talk. Later, he says that he found a talking dog but he's not sure if she's that talking dog.
- Friend to All Living Things: Fits his job.
- Nice Guy: very kind.
- When I Was Your Age...: Mentions how in her childhood, alphabet soup didn't have the entire alphabet.
Helen, TD, and Alice's teacher.
- Brainy Brunette: A smart teacher with brown hair.
- Deadpan Snarker: Mr. Parkington.
- Lovable Coward: Mrs. Parkington has moments of this and so does John.
- Mood-Swinger: John.
- Nosy Neighbor: Mrs. Parkington is quite nosy, but her husband generally isn't.
- Only Sane Man: Mrs. Parkington's imagination runs away with her and John is neurotic, meaning Mr Parkington has moments of this.
- The Shrink: John has his own therapist.
- Alliterative Name: Three "C"s.
- Broken Pedestal: Becomes this briefly in "Starstruck Martha" by the human fans when they find out his fear of flowers. They also hate him further when they don't fall for a fake bear scheme planned by Martha, Truman and Helen. Fortunately, he becomes a Rebuilt Pedestal after planning a scheme with a real bear named Bobo.
- The Danza: His name is "Carlo", which is the same name is his character.
- Lovable Coward: On TV he is brave, but he is actually this.
- Why Did It Have to Be Snakes?: He's afraid of flowers due to an incident.
- Shrinking Violet: Is a bit shy.
- Angry Guard Dog: Zigzags between this and Big Friendly Dog.
- Big, Friendly Dog: Zigzags between this and Angry Guard Dog.
- Berserk Button: Do not call him a bad dog or play a high-pitched frequency near him.
- Cloud Cuckoo Lander: Martha notes he's strange because he "plays with his steaks and eats his toys".
- Extreme Omnivore: Once tried to eat his toy.
- Jerk with a Heart of Gold: Mean, but not evil.
- Motor Mouth: Barks a lot.
- Big, Friendly Dog: Is large and friendly to other dogs, people and cats.
- The Ditz: Known for being this.
- Extreme Omnivore: Eats anything from diapers to pillows to rocks. This made him sick in "The Trouble With Teddy" and he needed an operation.
- The Klutz: Bumps into things a lot.
- Sick Episode: He gets sick in "The Trouble With Teddy".
The seven-year-old best friend of Truman.
- Genki Girl: Gender Inverted, he's a Genki boy.
- Lovable Coward: Is a bit of this.
- Not-So-Imaginary Friend: In his debut episode "Skits and Mr. Scruffles", he meets Skits, and, because Skits can't talk, decides to name him "Mr. Scruffles" as he doesn't know the name. His dad Terrence thinks that he made Skits/Mr. Scruffles up and when Skits tells Martha about Milo (in dog), she thinks he made Milo up because he was lonely.
- Potty Emergency: Averted, but he fakes this several times in "Milo's Reading Buddy".
- Short Guy With Glasses: He is shorter than the others, and wears glasses.
- Why Did It Have to Be Snakes?: Is afraid of reading in front of others.