Team Rocket's Jessie (and Wobbuffet) once left their partners in crime James and Meowth for love in Pokémon XY. The immensity of the moment underlines just how critical character arcs are in Pokémon and why another Team Rocket episode much earlier on in the anime was so effective.
Almost as weird as Ash somehow being gifted with eternal youth in the Pokémon anime, Team Rocket's Jessie and James have been mainstays in the long-running series since the very first episode. They are arguably just as recognizable in the anime as Ash, since appear at least once every episode. Shaking that formula in any way would not only be an innovative step that's uncharacteristic of Pokémon, but would drastically change the overall feel of the show, and altering the core three members of Team Rocket by negation definitely falls within this category.
So when Jessie leaves Team Rocket in the episode "A Fork in the Road! A Parting of the Ways!" of Pokémon XY, the show was at a major crossroads. After getting separated from James and Meowth, Jessie and Wobbuffet are saved by the handsome Dr. White after they are stunned by a Gloom. Jessie immediately falls for the doctor who has a mother Wobbuffet that is quite taken with Jessie's own Wobbuffet. After struggling with whether she could actually leave Team Rocket, a chance encounter with Ash actually causes her to bite the bullet and resign. Even James accepts her decision, though, tearfully.
How does James' special character arc measure up to Jessie's?
Of course, Jessie's departure only lasts for the span of this one episode because she learns that Dr. White is in love with another woman, but it's still a crucial moment in her overall development as a character. The irony behind Jessie's reason to leave isn't lost on longtime Pokémon fans who are aware that one of Jessie's lines in her and James' iconic intro is "to denounce the evils of truth and love." It's therefore devastating that, in the end, the one time Jessie goes against the Team Rocket code and gives love a chance she is spurned by love itself. For this will undoubtedly harden her will to continue rejecting love.
This moment actually harkens back to an episode in Pokémon's iconic first season when Team Rocket discovers that James ran away from his family, which is apparently rich and that he could inherit their estate if he married a certain woman. What made this particular James episode so deep for fans wasn't just that the story centered around a major Pokémon character, but because his betrothed looked exactly like Jessie, adding an intriguing new dynamic to their ostensibly platonic relationship. Just like how Jessie leaves Team Rocket to pursue what she and James have been denouncing for years in Pokémon XY, James is partnered with a woman who looks just like a person he has been running away from his whole life.
Although Pokémon is a show that usually doesn't build on past events like these and only uses them as fun one-offs, fans are still appreciative of these moments because they are associated with characters that some of them have been following for more than a decade. Jessie might never become a purveyor of anti-love sentiments beyond her intro, but it's still ironic that she chose love when she always introduces herself by saying "...to denounce the evils of truth and love." And James will never explore why he chose to work with a woman who looks exactly like the person whom he was meant to marry. But it's still a powerful image, especially for Pokémon's Team Rocket.