Flash
Voiced by: Tom Kenny
A blue alien who arrives on Earth to teach a tribe of cavemen called the Ronks to adapt technology.
- Aliens Speaking English: In spite of being an extraterrestrial, Flash already knows and speaks the English language.
- Exposed Extraterrestrials: He's an alien who doesn't wear clothes.
- Eye on a Stalk: His eyes are on stalks.
- Technology Uplift: Every episode, he tries to get the Ronks to adapt modern technology.
- Rubber Man: His body is frequently stretched in varying lengths and positions, though usually from others pulling at him and using him as a makeshift bond.
Walter
Voiced by: Rino Romano (pilot), Scott Whyte (series)
A caveman who is close to Flash along with his niece Mila.
- Cool Uncle: His niece looks up to him and he can come to her rescue when the chips are down.
- Top-Heavy Guy: It's anyone's guess how his small, spindly legs can support the weight of his huge frame.
Mila
Voiced by: Nika Futterman (pilot), Jessica DiCicco (series)
A little girl who is Walter's niece and Flash's other closest friend.
- Early Installment Character-Design Difference: The original four-minute pilot "Welcome to the Ronks" depicts her with lighter skin, blue eyes instead of green, no necklace, visible eyelashes, an orange dress instead of a yellow one and blonde hair worn in pigtails as opposed to black hair worn in a ponytail.
- Nephewism: She lives with her uncle Walter and it is never explained where her parents are.
- Puppy-Dog Eyes: "Game On" shows her getting her uncle to let her have her way by making big teary eyes.
Mormagnon
Voiced by: Charlie Adler
A Ronk who frequently plots to turn the rest of the tribe against Flash, as he's too accustomed to manipulating the Ronks through their superstitions to allow the tribe to make any significant technological advancements.
- Evil Luddite: As his goals are to undermine Flash's attempts at providing the Ronks with technological advancements for what are flagrantly self-serving reasons, he is this by definition.
- Fantastic Racism: He resents Flash for being an alien.
- Jerkass: He's an extremely rude and manipulative grump with nothing better to do than try and turn the rest of the Ronks against Flash by using underhanded methods to discredit Flash's ideas.
- Villainous Glutton: He will often instruct the Ronks to make offerings to the gods just so he'd amass a huge pile of food to eat.
Godzi
Voiced by: Dee Bradley Baker (episodes 1-13), Billy West (episodes 14-52)
Mormagnon's pet lizard.
- Hollywood Chameleons: He can blend in with his surroundings like chameleons do in fiction.
- Speech-Impaired Animal: Godzi can't talk and can only communicate through unintelligible sounds.
Mama
Voiced by: Cree Summer
The female chief of the Ronks.
- Miniature Senior Citizens: She appears to be elderly and is barely taller than Mila.
- Reasonable Authority Figure: For the most part, she's willing to hear Flash's ideas out and will renege a punishment after she's convinced the offender has made up for what they did wrong.
Rob
Voiced by: Charlie Adler
A red alien who is Flash's rival.
- Aliens Speaking English: Like Flash, he's an alien who can already speak English with no explanation.
- Exposed Extraterrestrials: Like Flash, he's an alien who doesn't wear clothes.
- Jerkass: He's a completely arrogant a-hole who loves mocking Flash.
- The Rival: He is Flash's rival in helping races on other planets develop their civilizations faster.
