Ash’s Pikachu in Pokémon seems to have been his journey-long partner since Episode 1 of the anime, with barely any episodes without him, rather than just being the series’ mascot.
But a fast look at the stats of the games will tell you that Pikachu is not an exceptionally powerful Pokémon with average abilities that almost double when he evolves to Raichu—so why is he not? With episodes and multiple Thunderstones available for over 20 years, why didn’t Ash evolve Pikachu into his stronger, more spectacular, and final form?
The response is simple: it was never his decision to make, in Ash’s view — it was Pikachu’s.
As a human being that never evolves, Ash has no reference frame for what evolution is like, other than external observation.
As such, he has never trained a Pokémon in the series with the intention of pushing it to evolve, nor has he concentrated on evolution as a way to become more powerful.
Although he is always pleased when his Pokémon evolve, he always respects their own decisions to remain the same, such as when Bulbasaur chose not to evolve in “Bulbasaur’s Mysterious Garden.” in Season 1, Episode 48.
Why does Pikachu not want to evolve, then? He’s still on the side of Ash, and getting stronger for the sake of his mentor is something he’s done time and time again, whether it’s practicing new moves like Iron Tail or Volt Tackle, or wholeheartedly engaging in Ash’s unorthodox training techniques.
The first time Pikachu was faced with this option was in Season 1, Episode 14, “Electric Shock Showdown,” where a well-meaning Nurse Joy offers Ash a Thunderstone after being soundly pounded by Lt. Surge’s Raichu, and both advise and warn him that evolution is irreversible.
It doesn’t take long for Ash, who had only one Pokémon (his Butterfree) to evolve to give Pikachu the option at this stage.
In his mind, the fear of change is evident—and when Pikachu swings the evolutionary stone skillfully out of Ash’s side, the two decide to grow stronger as they are—together.
The two are able to prove their mettle and beat Surge for Ash’s first truly won badge by taking advantage of the rapid evolution of Lt. Surge’s Raichu.
But that’s not the end of the story—revealed it’s that Ash held the Thunderstone in Season 11, Episode 20, “Pika and Goliath” just in case Pikachu had to change his mind about the whole thing of “evolution”.
This has its own sweetness, as it illustrates that Ash considers the feelings of Pikachu not only in the present, but in the future, and is ready to encourage Pikachu to evolve at any moment—if that’s what Pikachu wants.
Ash offers the choice to Pikachu once again when Pikachu is again defeated by a Raichu — this time with the caveat that this Raichu knows Hyper Beam, something Pikachu can’t understand until he evolves.
Pikachu displays contempt for the Thunder Stone at first, but later he looks more seriously at it, before the show cuts away to Ash returning to the bedside of Pikachu—to find no Pikachu—and no stone.
When they eventually locate the Pokémon mouse, he’s just practicing. Team Rocket seems to have robbed the stone and promoted Ash that Pikachu doesn’t need to alter to beat the new foe.
They turn out to be right, predictably (and get to make a pretty penny selling the stone!).
This does not explain Pikachu’s aversion to evolution, but a subtle explanation may have finally been offered by Pokemon Journeys.
“Enter Pikachu” revealed in Season 23, Episode 1, that Pikachu was originally a Pichu and lived with a Kangaskhan community that developed into a family.
Pichu wanted to leave after realizing the pressure he was putting on them, being borne alongside a baby Kangaskhan who was continually outgrowing the pouch of his mother—and his feelings of gratitude led him to turn into Pikachu.
Perhaps the reason Pikachu doesn’t want to evolve is that “evolving” is synonymous with saying goodbye to him—and he’s not prepared to leave Ash.