I figured it out as a kid, but I also played the first TMNT on NES before it, which has a similar character changing and item holding mechanic. Though yeah, knowing which cabins let you change characters and how to pass items, how the medicine works, you need the manual.

The big issue with it is it's just hard as balls even once you figure that stuff out, and how to properly navigate the map. It is a game that wants to encourage you to explore but punishes you for it because Jason won't fuck off for five fucking minutes while I am lost in this cave.