Ash Ketchum has been the protagonist of the Pokémon anime since 1997. This eternal ten-year-old won't rest until he has become a Pokémon master by defeating as many strong trainers as he can. To that end, he travels to various regions in the Pokémon world, accompanied by his first Pokémon and best friend, Pikachu.
Along with picking up new Pokémon, Ash has a habit of finding new human friends in each region. Folks from all levels of skill and background are drawn to the plucky protagonist and travel with him for one reason or another. The best of them bring unique skills and personalities that complement Ash's and leave strong impacts.
Misty
Tired of being overshadowed by her older sisters, Misty left the Cerulean City gym to prove herself as a master of water-type Pokémon. Unfortunately, she ran into Ash and Pikachu, who took her bike to outrun a flock of Spearow. After Pikachu wrecked her bike while defeating the flock, Misty refused to leave Ash until he got her a replacement.
While her motivation isn't the strongest, Misty makes up for it in personality. While she has a fierce temper when angered, she is generally kind and even motherly, as seen with how she raised her Togepi. She also undergoes a strong personal arc involving Gyarados: originally the one water Pokémon Misty hates, she bonded with one that evolved from a Magikarp in her gym after leaving Ash, eventually unlocking the power of Mega Evolution.
Brock
When his father left to pursue his own interests, Brock was forced to take over the Pewder City Gym and be the caretaker of his nine younger siblings. Eventually his father returned to help Ash win his first badge and resume his position as gym leader. This left Brock free to pursue his true passion of Pokémon care with Ash.
Brock is one of Ash's most popular traveling companions thanks to his running gags and heart. When not falling for every pretty girl he sees, he is a genuinely kind and considerate soul who isn't afraid to help any Pokémon he sees, regardless of the situation. Originally he wanted to be a Pokémon Breeder, but after traveling to four different regions with Ash, he realized his talents were best suited as a doctor.
Tracey
After arriving in the Orange Islands, Brock fell head over heels for Professor Ivy and left Ash and Misty. They would soon become a trio again when they met Tracey, an artist who loves to sketch Pokémon. After following the group to Kanto, Tracey would become the assistant to his icon, Professor Oak.
While not a bad character, if a little one-note, there was confusion at the time why Tracey was replacing Brock. According to the anime's then art director, Masamitsu Hidaka, it was done to give American audiences a more relatable character. However, fans had already grown to love Brock, so he rejoined the group in time for Johto.
May
Due to a poor experience in her youth with Tentacool, May didn't have the highest opinion of Pokémon. She only accepted a Torchic as her starter so she could travel the Hoenn region. However, she soon found a love for Pokémon contests, and with Ash acting as her trainer, worked to be the best coordinator she could be.
May offered Ash a chance to take on the mentor role and show how far he'd developed in his journey. Her personality also complimented his, being kind and considering of others, while still getting into spats with her younger brother, Max. However, woe to anyone who tried to steal May's food, as they would feel the full force of her wrath.
Dawn
Dawn knew from an early age that, when she was old enough to get her own Pokémon, she would follow in her mother's footsteps and be a Pokémon coordinator. Much like Ash, she had some rocky beginnings with her Piplup until she saved his life, and even saw a glimpse of a legendary Pokémon. Little did she know, she would end up helping Ash to save that same legendary from the evil Team Galactic.
While at first glance she appears like a more traditionally feminine version of May, there is enough to Dawn to make her a separate character. Since she knew her goal from the beginning, she starts off more confident than May, to the point of overconfidence in her skill. The inevitable losses that she experiences lead to an arc focused on humility and picking yourself up, which helps her bond with her disobedient Mamoswine.
Clemont
A child genius, Clemont works in Lumios City as the electric-type Gym Leader. However, he found himself overwhelmed and with no free time, so he invented a robot to run the gym for him. Predictably, the robot took its job too seriously, and Clemont was ejected from his own gym until Ash came and helped.
Clemont's intelligence and perseverance are his greatest traits. While his inventions rarely work as intended, he always picks himself back up after every failure and tries again, with his catchphrase, "The future is now, thanks to science!" He is also a caring brother to his younger sister, Bonnie, and caught her Dedene partner for her.
Serena
When she was young, Serena attended a Pokémon summer camp in Kanto, where Ash saved her from a Poliwag. Years later, she saw Ash in Lumios City, and decided to follow after him with her new Fennekin. With her encouragement, Ash won his first Kalos gym badge, and Serena decided to join in his adventures.
As with Ash's previous female companions, Serena's arc sees Ash helping another young trainer find their passion. However, Serena started off even further behind Dawn and May, as she knew very little about what she wanted to do with her life. This makes her overall journey with Ash more impactful, as she truly tries a bit of everything until she finds what she likes, and ends up leaving him in the end to pursue her dream.
Kiawe
During the Sun and Moon arc, Ash is enrolled in the Pokémon School on Melemele Island. Among his classmates is Kiawe, a fire-type trainer from the neighbouring Akala Island. He hopes to one day fill his grandfather's shoes as the Kahuna of Akala's Island Challenge.
Kiawe is already an experienced trainer upon meeting Ash, and had a Z-ring entrusted to him by Akala's current Kahuna, Olivia. He tries to present himself as a stoic and confident trainer, but he's also quick to show his emotional side to his friends. When not at the school or training with Ash, he works hard on his family's farm and tries to set a good example to his younger sister.
Lillie
Due to a traumatic event in her past, Lillie is unable to touch Pokémon. Nevertheless, she enrolls in the Pokémon School in the hopes of overcoming her fear and becoming a Pokémon researcher. Unfortunately, her past trauma and complicated family-life both catch up to her and cause trouble for her friends.
Lillie's arc in the show does justice to her game counterpart, who is considered one of the best characters in the core games. Watching her slowly work through her trauma is inspiring and results in a heartfelt conclusion. Besides that, her personality is fun as the polite rich girl who knows the answer to most-every question.
Goh
As a child at summer camp, Goh's life changed when he saw the mythical Pokémon, Mew, and vowed that he would catch it. Unlike Ash, Goh isn't a big fan of Pokémon battles. Instead, he wants to complete his Pokedex and catch at least one of every Pokémon while he waits for Mew to return.
Goh was created as a stand-in for fans of the mobile game, Pokémon GO, where the objective is to catch as many Pokémon as possible. This results in him becoming a sort of jack of all trade character, who can have a Pokémon for almost any situation he and Ash find themselves in. What's more, he cares for each one and works directly with them to overcome personal flaws.