Television

• Scrappy Doo
• Poochie (on The Simpsons but he was a parody)
• Pretty much any child character added halfway through the show's run

Edit: The character of Jenny on Unhappily Ever After. She was played by Stephanie Hodge, and she was supposed to be the star of the show. Everyone hated her character, and she eventually got written off, came back as a ghost, and then abruptly left again.

The series was initially written as a starring vehicle for Hodge, whose character Jennifer was the focus of the first few episodes. However, the series soon turned its focus to Jack Malloy, a schizophrenic, alcoholic, and lazy husband who was kicked out of the house in the pilot episode. ... By the show's third season, Tiffany Malloy had become the breakout character, and Cox became the de facto co-star of the show along with Pierson.

In the fourth season, producers decided to kill off Jennifer's character, but returned her as a ghost. After continuing to torment her family as a poltergeist known as "Dead Mom", the Malloys realize that the show doesn't work without Jennifer as their common enemy. Jennifer is then brought back to life after a metafictional sequence, commonly used in the show, in which a network executive enters the house and explains to the characters that, due to the jokes no longer being funny, "Dead Mom" is no longer dead. Jennifer then returns and is overjoyed to be restored to life, until she sees the gigantic mess the family left for her to clean up. Jennifer throws a massive temper tantrum and vows to take terrible revenge. Nevertheless, Hodge decided to leave the show anyway, and several episodes after Jennifer's "resurrection", she eloped with a lesbian lover and was never seen again.

-- https://en.wikipedia.org/wiki/Unhappily_Ever_After