Justify The Existence

Problem: You have a Fan Service Doll whose unsure existence is starting to get on your audience's nerves.

Solution: Suddenly make the Doll participate in battle (making her valuable to the Good Guys) or give her a goal in life that becomes the objective in a side story.

When the writers need to come up for a reason why someone or something is in the story. Without a reason, that entity in question becomes a waste of space and should be kicked from the story. This only ever happens to people who are in the Main Cast (most notably Useless Secondaries) and sometimes Secondary Cast.