In the Pokémon cartoon series, one of the most powerful creatures on Ash's early roster was Charizard, yet Ash would eventually decide to part ways with the fan favorite Pokémon, leaving him behind at Professor Oak's lab at the end of season 16 back in 2013. Charizard's journey with Ash began all the way back in season one of the series when he was still a Charmander. Abandoned by the neglectful trainer Damian, the young Charmander was rescued by Ash, where the creature's journey would prove to be one of the most compelling in the run of the series.
When Charmander first joined Ash, the Pokémon was grateful to have a master that now cared for him, so he was incredibly loyal and obedient. When he evolved into Charmeleon though, things began to change. While Ash was still growing as a trainer, Charmeleon grew more powerful and arrogant, so he began to disrespect Ash. When Charmeleon became a Charizard, the defiance only grew, making the fire Pokémon extremely difficult to manage. Things finally changed though when Charizard was badly defeated by a Poliwrath and Ash dutifully nursed him back to health. After that, Charizard again became one of Ash's most loyal Pokémon.
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Being only the fifth Pokémon Ash ever caught, by season 16 of the cartoon, Charizard had been with Ash a very long time (even if he did, on occasion, go to live with wild Charizards in the Charicific Valley). During season 16's Unova League battles, Charizard was a valuable asset, defeating a Dragonite as well as helping to fend off attacks by Team Rocket. While Ash didn't win in the Unova Leagues, he grew as a trainer and placed in the top eight. Following the competition, he returned home to Pallet town to visit his mother and Professor Oak. It was there that he renewed his vow to become a Pokémon Master and decided to leave most of his Pokémon — including Charizard — with Professor Oak. With the long relationship Ash and Charizard have had, this may have been surprising, but it made sense for Ash's character evolution.
While Ash did well in the Unova Leagues, he still came up short and he's hardly a character to settle. So, after a brief respite with his mother, he rededicated himself to becoming a true Pokémon Master, which means winning in a league and meeting every Pokémon there is. The next season of the series saw Ash arrive in the Kalos Region, where there were new, unfamiliar Pokémon for him to interact with. In addition to that, Ash only had Pikachu by his side, which meant that he had to capture and train a whole new set of Pokémon. Ash does this before entering every new region as a way to be sure that he continues to grow as a trainer. Were he to always rely on well-trained, powerful Pokémon like Charizard, they may become a crutch for him, but by starting anew, Ash ensures that he'll keep challenging himself.
As for Charizard, chilling at Professor Oak's place isn't so bad, as many of Ash's other Pokémon are there as well. Among them is Ash's Pignite, who Charizard bonded with during the Unova Leagues. Charizard may not be at Oak's lab for much longer either, as he was spotted in a recent trailer for some upcoming episodes of the cartoon. Back in 2019, Pokémon was rebranded as Pokémon Journeys: The Series, which follows Ash as he explores regions that he's visited before. Last October, Charizard was spotted among a crowd of many of Ash's Pokémon, suggesting he should be appearing pretty soon.
Whether or not Charizard's return is just a cameo remains to be seen, but it would certainly be nice to see him in action again. While Ash staying sharp is certainly important, it's been eight years since Ash's Charizard last appeared in the series, which means that the fire-breathing, fan-favorite fire Pokémon is certainly overdue for another epic showdown.