I assume that's probably the one right after the pumpkin miniboss. Ok, big spoilers here, read at your own risk.
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Depends on whether or not you've visited Proto Man twice. He lurks somewhere in Cloud Man's stage and Turbo Man's stage. If you find him in both and go back, you fight Proto Man as a miniboss. If you win, he gives you his Proto Shield. (Not ...
The
***Spoiler Warning?***
Rush Adaptor? It gives you a "double jump", sorta, and the charge fist is real powerful (even more so if you find the Power Knuckle in Turbo Man's stage [Rush Search needed, look for a horizontal traffic light]).