Characters That Had Their Series Hijacked From Them

Any series where the character we start with has their importance and focus diminished substantially as time passes, for better or worse.

For a good example, for all intents and purposes, Gon is no longer the main character of Hunter X Hunter. You could argue it became that way immediately after he killed Pitou, but it's definitely the case now. The plot hasn't had anything to do with Gon for a while, and HxH fans seem to be content with Kurapika picking up the mantle.

For an example that makes me angry and may or may not be the reason this post exists, Cobra Kai. The premise of the show is following the adult life of Johnny Lawrence, the bully from the first Karate Kid who Daniel Larusso defeats in the tournament. His life went to shit after that loss, and he's a washed up loser asshole deadbeat with nothing at the beginning of the series.

The biggest attraction to the series is watching Johnny become a mentor to a new generation of kids, fix his life, and become a better person while struggling with the highs and lows of what Cobra Kai instilled in him. The problem is that Daniel's issues start to eclipse a healthy amount of the rest of the plot. The more the show focuses on Daniel, the worse it tangibly becomes. It doesn't help that he's a massive hypocrite obsessed with another man's legacy, and unlike Johnny, Daniel is almost never made to actually confront his worst traits.