It's impossible to imagine Ash getting very far in the Indigo League without the help of his trusty companions Brock and Misty. While perhaps stealing and then wrecking someone's bike to make a lifelong friend isn't actually very good advice, this is what got the Cerulean City gym leader and Water Pokemon master Misty along for the ride. Brock joined up in the original crew after his father took his place as the Pewter City gym leader leaving him free to travel.
However, the triumphant trio was not destined to be together forever. Despite Brock holding the record for being the second-most seen character in episodes of the anime, both he and Misty drifted away to do their own things while Ash adventured across nine separate regions and the Orange Archipelago. So what exactly did they get up to in their time off-screen?
Brock In The Orange Island Saga
The first time fans saw Brock leave was when Ash decided to explore the Orange Archipelago. During this time, Brock was replaced by Tracey due to fears of the localization team that Brock might be perceived as a racist caricature by American audiences.
To write him out of the show, they had him decide to stay with Professor Ivy on Valencia Island as he had absolutely fallen in love with her. Outside a couple of brief cameos of the two together in Pokemon the Movie 2000, not much more is known about Brock and Ivy's time together. In fact, Brock is particularly vague about what happened, curling up into a small ball and requesting no one say her name whenever she is mentioned after Misty and Ash meet up with him once more following their return to Kanto.
Fans have speculated for a long time about what exactly happened, but there is no official explanation yet. A potential explanation comes from a theory presented by the original head writer of the Pokemon anime, Takeshi Shudo in his book Pocket Monsters: The Animation. He personally believed that Brock was particularly affected by the rejection because Professor Ivy was a lesbian, meaning he never actually had a chance to begin with, unlike the other women he's constantly seen flirting with.
Brock And Misty After Johto
While Brock returned to join Ash and the squad as they traveled through Johto, it was not meant to last forever. In the 273rd episode of the anime, Gotta Catch Ya Later!, Brock, Misty, and Ash all went their separate ways. Misty was finally able to get her broken bike fixed and used it to return to Cerulean City Gym, as she returned to run it while her sisters went on a world tour. Even after her sisters' return, she seems to remain the primary Gym Leader there, with only her sister Daisy staying around.
Brock also returned home, only to be surprised that his mother had returned to the family after so much time, turning the entire Pewter City gym into a water Pokemon den. In the end, Brock was able to defeat her in a Pokemon battle "for the honor of all rock-types" before returning to Ash to travel with him across the Hoenn region.
Hoenn And Sinnoh
While Brock traveled with Ash throughout the entire Hoenn region, Misty did make a handful of appearances in the region, including the heartbreaking episode, A Togepi Mirage, where her beloved Togepi evolved into Togetic and decided to stay and protect the Togepi Paradise. She also stayed around to watch Ash compete in the Ever Grande Conference before returning home. However, she was also invited to visit the Mirage Mansion during The Mastermind of Mirage Pokemon TV special. It is assumed that Misty is working at home as a Gym Leader whenever she isn't on-screen.
As for Brock, he traveled with Ash throughout all of Hoenn, briefly returning home before continuing their adventures across Sinnoh. It was only after his adventures there that Brock decided that he wanted to become a Pokemon doctor and decided to stay in Kanto to study.
In an episode that never received an English dub, it was revealed that Brock had been working hard to help support the Pewter City Pokemon Center while waiting to attend Pokemon medical school. He even helped his younger brother Forest become the new gym leader for the town after it was revealed their father never submitted the proper paperwork to transfer the title to him. After Brock gets defeated by a Gym inspector, he is able to convince her to give Forest a try at being certified by showing off his impressive medical knowledge of Pokemon.
Alola and Beyond
While Brock did have a single episode appearance in Unova where it was revealed that he had apprenticed under a Pokemon doctor in Johto as part of his training, no updates on the original crew were given until they all reunited in the Alola region.
Viewers learned that Misty had been hard at work as a Gym Leader, with so many trainers challenging her that she needed a break. She ended up leaving her sisters to help keep things running while she traveled with Ash across the islands. Both she and Brock returned home after a fateful encounter with Island Kahuna Olivia.
Currently, there is no other information on what Misty and Brock have been up to. It is assumed that Misty returned to work at the Cerulean City Gym and that Brock is now working as a Pokemon Doctor somewhere in Kanto. It does look like Brock will be returning in the Pokemon Journeys anime so fans may finally get an update on his adventures then.