I have encountered contradicting information on this matter when browsing the internet. Some sources say:
Booster 0.8: Received from the professor in the labyrinth, if you follow his downward path. Click jump two times to fly. Note: Unable to upgrade to Booster 2.0 on easy mode in Cave Story+.
However, other sources state:
You basically need two things to get the Best Ending. You need the Booster 2.0 and you need Curly.
Does that mean that the best ending cannot be accessed in Easy Mode?