Brock wanted to be the greatest Pokémon Breeder (he is now following his destiny as a Pokémon Doctor), and gives up his title of Gym Leader after Ash won his first Kanto League Gym Badge from him. He was first seen taking care of his nine siblings.
After Ash loses to Brock, a stranger (Brock's father) offers to help make his Pikachu stronger. Pikachu (after being hooked up to a local hydro-power station) becomes stronger and manages to defeat Geodude, but Onix remains too strong. Onix has Pikachu in a body bind; however, Pikachu's previous electric attack ravaged the gym, which set off the sprinklers, weakening his Onix. Ash declines the badge on the grounds that he unfairly beat Brock, but Brock later catches up to him and presents to him the badge, saying that he wants Ash to fulfill his dream of becoming the best. The stranger that first helped Ash reveals himself to be Brock's father, Flint, who has a tendency to not notify people it is him. Brock is now free to train himself, and joins Ash and Misty on their journey.
Since then, he travels with Ash and Misty on his many adventures. In the Orange Islands series, he leaves the team to join Professor Felina Ivy, but mysteriously returns to Pallet Town when Ash wins the Orange League Championship. When asked what happened, Brock just rolled up into a ball and replied: "I don't wanna talk about it." or "Don't mention that name." every time someone mentions Professor Ivy. Rejoining Ash and Misty, Brock travels to Johto, where he continues to help Ash and Misty. Upon the conclusion of the Silver Conference, the three part ways, Brock heading back to Pewter City. It is there (in an episode of Pokémon Chronicles) that he discovers his estranged mother Lola (Mizuho in the original), who, because she is a Water-type Pokémon Trainer, has ruined the reputation of the Gym he defended so long ago and had ended up filling the gym with water (causing Brock to comically freak out and keeps repeating "A LAKE!!!" when he first sees it).
By defeating his mother, he restores the Gym's reputation, and proceeds to leave all his Pokémon (except Forretress) to his younger brother Forrest, before departing for the Hoenn region, where he catches up with Ash and his two new friends, May and Max. The group travel across Hoenn and then return to Kanto to participate in the Battle Frontier. The group went their separate ways and Brock met up with Ash in Sinnoh. While there, they meet a new coordinator named Dawn and her partner Piplup, who joins them on their journey through Sinnoh .
During this time, both Brock's father, Flint (Munō in the original, who was also Brock's predecessor as Gym Leader), ran off again, and his oldest younger brother Forrest (Jirō in the original) took the role of Gym Leader in Pewter City, as well as the responsibility of caring for his many other younger siblings. Brock is the oldest of ten siblings, with five brothers and four sisters.
Brock is always well-equipped, and is sometimes the voice of reason when Ash and Misty (and later, May and Dawn) get into an argument. He is also hardworking and is always willing to help others. Coupled with his experiences in taking care of others, Brock often prepares food and does most of the chores for the main characters. Brock is also exceptionally good at making food for Pokemon.
At the end of the Pokémon: Diamond and Pearl series, Brock aspires to become a Pokémon Doctor, and work alongside Nurse Joy. After departing from Ash and Dawn, Brock returns home to Pewter City to train to become a Pokémon Doctor.
In the Black and White Special he encounters Cilan in a small town. He helps treat Pansage after it got hurt fighting a Gyrados. The two form of fast friendship and work together as they help Gyrados. As they part they both realize they never asked for each others name.
Over the course of the series, there have come to be a couple of recurring gags related to Brock that have become quite prominent.
One of the most notable running gags in the anime is that Brock makes advances on virtually/love sick crazy on every girl his age or older, and is usually met by rejection, confusion, or being dragged away from them by Misty, Max, or his Pokémon, Croagunk. One of several exceptions was Professor Ivy, the Pokémon Professor from the Orange Islands, with whom he spent some time away from the show. He later returns but refuses to explain why he left her, simply saying "Don't mention that name!"Pike Queen Lucy of the Battle Frontier in Tohjo Falls seems to have a liking towards Brock (partially because she has a soft spot for Pokémon with squinty eyes) almost traveling with him, before Max interrupts. Other girls who had feelings for Brock include Wilhelmina, Temacu, Natalie, Autumn, Narissa, Lizabeth and Holly.
His crushes include the various Nurse Joy's and Officer Jenny's, which he can differentiate with ease based on very perceptive but minute observations. Because of this, he can spot impostor Joy's and Jenny's instantly. Other crushes are: Rhonda, Rhoda.
Another one of the notable running gags is how Brock is forced to entertain an audience while other characters retrieve something which would typically be another Pokémon that would have been putting on a show for the current audience that has likely been stolen by Team Rocket. During the cases in which he does,such as he repeats one song (usually an English version of "Takeshi no Paradise", one of the Japanese ending themes) ad nauseam to the point that he is either booed offstage or loses interest in singing he reveals that he sings this song over and over is because its the only song he knows. After Ash finished the Sinnoh league Ash and Brock left back to Kanto and went their separate ways. In the Pokémon DP Special Episode 2, Brock is shown studying to become a Pokémon Doctor, and even has photos of Ash, Pikachu, Misty, May, Max, Dawn and Piplup, reminding himself of how much he misses all of them.
Brock has only met his recently evolved Steelix once. Evolving while under the control of his brother Forest, it remains loyal to Brock and follows his commands at will. It has gained strength after the evolution and remains to be an amazing Pokémon at the Pewter Gym for Brock to call upon whenever he needs it. Brock's Steelix debuted in Showdown In Pewter City as an Onix and in Grading Spaces as a Steelix.
How Brock obtained Geodude is unknown, however Geodude has always been a vital member of Brock's team, always being there for battle or for certain contests. Geodude is an unfailingly fun loving Pokémon that is known to have fun with Pikachu and the other Pokémon on many occasions, however it was seldom seen. When Brock left for Hoenn, he also left Geodude in Forest's care. Brock's Geodude debuted in Showdown In Pewter City.
Evolving due to Brock's immense care and bond with it while in battle with Team Rocket, Crobat is a Pokémon that follows the orders of its trainer. With its four wings, it is a super fast flying Pokémon that has been known to help Brock out of jams on several occasions. When Brock left Pewter Gym for Hoenn, he left Crobat under the possession of his brother Forrest, however Crobat is overjoyed to see Brock whenever he returns to the gym. Brock's Crobat debuted in Clefairy and the Moon Stone as a Zubat, 'Hassle in the Castle as a Golbat and Control Freak as a Crobat.
After evolving from Pineco, Forretress became Brock's most commonly used Pokémon. It got over its habit of exploding when near Brock and became a powerhouse. Forretress was the only one of Brock's Pokémon that he took to Hoenn with him and resided as a vital member of his team until Sinnoh. Brock's Forretress debuted in Entei at Your Own Risk as a Forretress.
After finding a Water Stone and evolving, Ludicolo became a more friendly and energetic Pokémon and is known to dance on occasion. It has not been used in battle often so it's true battling power is still unknown. When Brock returned home from Hoenn, he decided to leave Ludicolo at the gym to entertain his family.
After evolving, Marshtomp became a strong Pokémon often used to settle feuds or just in simple battle. Marshtomp is a well mannered Pokémon which takes after it's trainer a little bit in regards to romance. Brock notably used Marshtomp in a Pokémon Contest against May. Brock's Marshtomp debuted in A Chip Off the Old Brock.
Having evolved from Bonsly, Sudowoodo has shed it's child like persona and has become a powerful Pokémon of Brock's team. With Powerful attacks including Mimic and Headbutt, Sudowoodo is the powerhouse of Brock's team.
Croagunk is a rather stubborn Pokémon. Meeting him at an unofficial Pokémon Gym ran by Team Rocket, Brock and Croagunk quickly gained a close friendship. Eventually teaming up together to get rid of Team Rocket, Croagunk decided to join Brock's team. It has many powerful fighting and poison attacks and will become a vital asset to Brock's team. When Brock sees a girl, Croagunk uses Poison Jab on him.
While on the boat to Twinleaf Town, Brock's Happiny evolved into Chansey while trying to help a Pichu. It is currently unknown if Chansey has kept the strength it had as a Happiny, but it knows the move Softboiled which it used to heal the Pokémon on the boat.
Vulpix was an incredibly particular Pokémon, only eating food that either it's original trainer, Suzy, or Brock made. Through seeing the care that Brock went into in looking after Pokémon, she decided to let Brock look after it. Brock was often seen grooming it and sometimes used it in battle, mostly however in training with Ash. Vulpix, while a little spoiled, did enjoy playing about with the other Pokémon. Eventually Brock met up with Suzy again and decided that Suzy should keep Vulpix again.
In the episode, "A Crowning Achievement", Brock actually temporarily opens his eyes in a flash second to try to cheer up Larvitar.
Brock's eyes are rarely open, excluding the examples shown.
At the beginning of the anime, Brock didn't show a lot of interest in girls. After a while, he finally shows it.
Brock is the only character who appears in all the seasons except the Best Whishes seasons, although he only appears in flashbacks made from Dawn and Ash.
As seen in the episode "The Taming of The Shroomish", Brock is apparently afraid of ghosts. While he and May were shopping, she heard of a haunted mansion. He was terrified just hearing about it, and, while May was overjoyed, she literally had to drag him there. Whenever he heard somebody talking about ghosts, he leaned away. He was afraid of the mansion, and showed dismay and horror when they talked about advancing inside. He was fine once he found out that the ghosts were actually Shroomish, though. He was also fine on DP090: Ghoul Daze!, among other episodes.
Brock's real dream is to become a Pokémon doctor. However in "Jirachi: Wish Maker", he said he wanted to become a magician because he could make "beautiful girls appear out of thin air".
It is said that Brock was taken out and replaced with Tracy due to the notion that he would be racially insensitive to Pokemons growing world wide audieance. When it was revealed that wasn't true they put him back in for Johto.
Brock holding a rice ball (known as a "Donut" in the english dub)