False protagonist
A false protagonist is when someone seems like the most important character in a story, but later on shows that they are not for different reasons, (becomes a lesser character, becomes the antagonist, dies, etc.) An example is Roxas in Kingdom Hearts II.
False Protagonist Media
Aladdin and the sorcerer, in the story Aladdin. The story starts by following the sorcerer as he seeks a magic lamp, but Aladdin is later revealed as the story's protagonist.