Manga in which the protagonist can enter stories The protagonist enters a story and becomes the replacement for the original protagonist of that story. After each arc, he uses an element of a previous story in order to solve the current story's world's problems. It's still quite new so it should be under 50 chapters.

I read it maybe a half a year ago. Does anyone what manga this might be?
It could be KINGDOM HEARTS, and is actually based on a PlayStation 2 game that came out first.

In the main story, Sora (the protagonist) travels among other worlds. And in each world, he transforms and blends in to resemble the world's characters.
Also in every world, there is a problem that only he could help to fix.

In the first season of this series, there are 43 chapters