\-Clara Oswald - Doctor Who: The character was introduced as a kind of plot device becoming a companion later, and somehow, the showrunner Stephen Moffat turned her in the most important companion the Doctor ever had (I think they even say it in the show), she became really proud, even struggling with the Doctor, putting at his same level and even being rewarded with her own TARDIS.

\-Katherine Brewster - Terminator 3: Introduced as the future wife of John Connor, the character feels out of nowhere and the plot undermines John's importance to boost her, turning her in a main piece of the plot with an almost secondary John.

\-Clan Uchiha - Naruto: Kishimoto was so obsessed with this people that turned the story about Naruto in a story that turns around this people, making them the most special ninjas ever existed, fogeting about the rest of clans, and so, God saves you to face one of them because they gonna win only for being Uchihas. Specially damaging was the obsession protagonist had with his criminal Uchiha "friend".

\-Albert Wesker - Resident Evil series: Introduced as a traitor and villain in the first game, the plot resurrects him in a unnecessary way only to make him to steal the importance the pharmaceutical company Umbrella had as the main antagonist of the series, even discarding it and its fundator as extras, to sell Wesker as the real master mind of the story.

\-Zero - Megaman series: The creator of the character, Keiji Inafune, became so obsessed with the character that made the story of X to turn around, and only around his creation, turning X in an extra in his own story with almost no development, damaging the plot permanently.

[Hated trope] When a character gains a plot's importance that wasn't supposed to have and damages the entire story