Ash encounters many different Pokémon on his quest to become the greatest there ever was. One recurring character in particular has truly made its mark since the original anime and has long followed Ash on his long and winding journey.
Jigglypuff shows up without fail in every new region, singing the team to sleep and drawing on their faces out of frustration. Why does it do this? Why does it put on a show only to sing to a sleeping audience?
The answer is validation, much like any human artist craves. Any social being thrives on validation, and Pokémon's Jigglypuff is no exception. In Episode 45, "The Song of Jigglypuff," Misty planned to catch this particular Pokémon until she realized it couldn't sing. The gang decides to find its lost voice, and eventually Brock heals Jigglypuff by giving it a berry. As thanks, it decides to sing a song for them -- but they all fall asleep almost immediately.
Angry that they didn't appreciate its special song, Jigglypuff grabs a marker from Ash's backpack and draws all over their faces while they are asleep. When they finally awake once again, Ash promises the insulted Jigglypuff that he will find someone to listen to its song without falling asleep. Thus, it is seen checking in with Ash throughout the different iterations of the Pokémon anime, making sure he is keeping his word.
This specific Jigglypuff seems to be quite the diva. As seen in Episode 149, "Tunnel Vision," it is obsessed with becoming a famous singer as it continues to follow the gang through the Johto Region -- to the point of not being able to depict reality from imagination. Jigglypuff can be seen throughout the series serenading everyone to sleep and protecting Ash's marker that it uses as a microphone.
In Pokémon the Series: Ruby and Sapphire, Jigglypuff finally got the audience it deserved after five years of being in the anime. In Episode 39 "A Poké-BLOCK Party," Jigglypuff comes across Alanna's Whismur. It possessed the Soundproof ability, which allowed it to listen to Jigglypuff's song without falling asleep. It is the only person or Pokémon to ever listen to the song in full, although it promptly fainted afterward due to exhaustion from a prior battle. While Ash's promise is finally fulfilled, Jigglypuff is seen again in Pokémon the Series: Sun and Moon singing its song to a sleeping cast, possibly pursuing its dream of being a famous singer.
Fans of the anime admire Jigglypuff for its tenacity and the novelty of the long-running gag has not been lost. One can only assume that eventually, Jigglypuff will become a famous singer across every region of the Pokémon universe. How it will achieve that incredible feat is unknown, but surely the determined diva will find a way.