I like the idea of combining the elements to create new characters, however I did not find a way do "undo" a fusion as such I got stuck on several locations (including the beginning). Also R2 jump or "yeet" kills the player instantly if there is no secondary character picked up. It also was very weird to have controller only controls, but mouse interaction for the menus.
I definitely see possibilities here, everything is just a tad to rough. (You are using unity, so perhaps multiplying your displacement with Time.deltaTime could fix your stuttering issues, if it is completely physics based make sure to move all calculations to FixedUpdate and all controller inputs to Update).
Keep it up!