This is centered around the adventures of Jannette Jey, an optimistic teenage girl; and her friends she calls her "posse", Luis Inez; and siblings Max and Cam Johnson. They travel the galaxy to discover various alien races and visit planets for their curiosity.
Captain Jannette Isabelle Ley (voiced by Cristina Pucelli) is the leader of her group of friends she calls her "posse", as well as captain of the Mark Infinity. One day, she had found the Mark Infinity crashed landed in the meadow fields outside her neighborhood and telling her posse about it, followed by activating it accidentally with them in it. Ever since that day, she and her friends have started traveling the galaxy.
She is reckless and optimistic on what she does, usually when it comes to being leader. She will usually start to listening to music on her mobile phone with wireless headphones wherever, either on Earth, on the Mark Infinity or on a various planets. She is also somewhat of a tomboy, as she is usually clipping her toenails with her teeth or take her shoes off along with her socks and puts them up on the Mark Infinity's control panel. In almost every episode, she'll be seen wearing different clothes.
Mosley chose Pucelli for the role of Jannette after picturing Pucelli's character Luan Loud (a character from Nickelodeon's The Loud House) with Star Butterfly (a character from Disney's other animated series Star vs. the Forces of Evil) and Mabel Pines (a character from Gravity Falls, another Disney series). Some of her character personality traits originate from some fictional characters from other media Mosley had seen; her habit of listening to various songs on her headphones is similar to Star-Lord from the Marvel Cinematic Universe and serving the role as the captain with a cocky attitude like Han Solo from the Star Wars franchise.
In each episode, she is usually seen wearing a different pair of clothes, similar to Mabel Pines from Gravity Falls and Star Butterfly from Star vs. the Forces of Evil.
Luis Romano Inez (voiced by Josh Keaton) is Jannette's best friend and secretly her love interest until the third season. He is supportive in Jannette's decisions, yet has worry for what happens to her. When traveling with her, he serves as her cadet He once dated his ex-girlfriend Leslie Harper before meeting Jannette.
Maximilian Benjamin Roberto "Max" Johnson (voiced by Kath Soucie) is the overweight troublemaking best friend of Luis and baby brother to his sister Cam. He usually pranks and teases his sister when he gets the chance and play app games on his phone in his free-time. He is stubborn and carefree, but he has no problem to stick up to his friends and especially sister. He has the job as Jannette's co-pilot when traveling space.
Mosley based his troublemaking personality off Bart Simpson from The Simpsons.
Camellia Louise "Cam" Johnson (voiced by Kari Wahlgren) is Max's older sister and Jannette's third member in her posse. She serves as the straight man to the team, only to be ignored by her friends; mostly by her brother. She and Max have a tough relationship and it only consists of fighting and arguing, but no matter what, they stick together despite their constant bickering. On Jannette's travels, she is the team's maintenence inspector.
Cam's character role is reminiscent to Family Guy's Meg Griffin; with being ignored by the people she's with, put into constant slapstick, being the older sister with an overweight brother, and usually picked on.
Wilson (voiced by Kevin Michael Richardson) is a Lanate alien species whose apart of Jannette's intergalatic adventures. Back on his home planet; Lanatane, he was an inventor for his people, but after introducing his superweapon to protect his kind, he was exiled due to their peace-centered lives. Jannette and her posses had found him afterwards and Jannette took him in. He now helps the team as their engineer and weapons specialist in creating new weapons and inventions on their adventures.
Mosley reluctantly chose Richardson for the character due to Gravity Falls and Star vs. the Forces of Evil having used him as an additional voice.
Queen Emprise I (voiced by Katey Sagal) is the leader of the Lanate race that wants to keep her planet in tranquility. She disproves of conflicts and battles, along with weapons.
The Mark Infinity's artificial intelligence, nicknamed A-I by Jannette (voiced by Frank Welker), is operated by the brain of the creator of the spacecraft, whom was an intergalactic superior alien being. His main purpose is alerting the posse and informing them about distress signals from various planets for them to save.
Constance Ley, neé Ann (voiced by Cristina Pucelli), is Jannette's mother and who has a stern personality and anger issues. Her relationship with her daughter is to the point that Jannette distances herself from her. She has a tendency to grasp her daughter by the wrist tightly to when she plans on leaving or wants to ignore her. She does not appreciate her spending time with her posse and her keeping secrets from her.
William "Bill" Ley (voiced by Josh Keaton) is Jannette's late father who, unlike his wife, respected his daughter's choices. He was easy-going and the most polite in the family. His relationship with her daughter had him and Jannette bonding over watching science fiction shows.
Mr. & Mrs. Inez
Nick (voiced by John Viener) and Penny Inez (voiced by Tara Strong) are Luis' parents. They are both wealthy but spoiled, unlike Luis. They barely pay any attention to their son and are more proud of his elder brother Herb. They tend to spend their days heading out of town to either motels to have time to themselves and attend the local yacht club
Larry & June Johnson
Larry Tiberius (voiced by Billy West) and June, neé Madge, Johnson (voiced by Christine Lakin) are Max and Cam's parents. Larry is lethargic and has anger issues, while June is benevolent and tranquil.
Barry Gordon High School
Principal Anthony Jarled (voiced by Ian Hecox) is the principal of Barry Gordon High who keeps a strict reputation around his school. The student he despises the most is Max, yet, his other student, Jannette, is known for being late and receiving tardy slips for that.
Katheryn "Kate" Hannigan (voiced by Courtney Miller) is the lead cheerleader to the Barry Gordon High and leader of her group called the Smash Pit, as well as a rival to Jannette; the two have been once friends in middle school, until Jannette had pulled an elaborate prank on her at the school talent show, resulting in the end of their friendship.
She is free-will minded and does as she pleases, even when it comes to threatening her parents to do whatever she wants despite their orders.
Ronald "Ron" Thompson (voiced by Shayne Topp) is Kate's boyfriend and member of her group the Smash Pit. He respects his girlfriend and goes along with her decisions. He is pegged as the class clown in the high school, a reference to Topp's character Jordan Schwarz, a main character from Smosh's "Most Annoying Kid" series.
Carly Chu Yang (voiced by Olivia Sui) is a foreign exchange student from China whose apart of the Smash Pit. She is very flamboyant and energetic, unlike the stern and arrogant Kate. A running gag with her is how Jannette, or any other character, usually brings up various Chinese stereotypes, much to her annoyance.
Chuck Goldman (voiced by Keith Leak Jr.) is the third member in the Smash Pit that Kate randomly picked, to which he joined because he wanted to be included in things. He is the only one in the group that is cautious about their actions, to which Kate retorts "Shut up, Chuck!".
Glen Whitshaw (voiced by Noah Grossman) is the fourth member of the Smash Pit who was an athletic champion in his prime. He enjoys taunting Jannette's posse on a daily basis.
The shapeshifter is an unknown extraterrestrial species that can shapeshift into any being it wants to become. The character made its debut in "Jannette & Jannette" and took Jannette's form after the posse's arrival on Muhfafsia. Its second appearance was in "Cool Hand Jan", when it was taking Jannette's form to sack revenge on her by thieving and kidnapping. Fortunately, it was apprehended by the posse and sent drifting into space stuck in one of their escape pods to capture it.
Lt. Aldrin Toomes (voiced by Jeff Bergman) was A-I's second in command who was responsible for placing A-I's brain in the Mark Infinity's main control module six years ago during their battle between the Zeta Reticulans. Now, unbeknownst to A-I, he is currently working with them, in exchange for his life.
Empress Quara (voiced by Daisy Ridley) is the leader of the Zeta Reticulan race whose main purpose in life is to wipe out the entire human race due to believing that they are inferior and half of their kind are incompetent.