This month, the Nintendo Switch version of Cave Story+ receives a free update that adds local two-player cooperative multiplayer to the game. Now, it has been revealed that playing through this mode with another person unlocks various costumes for the second player. For example, going through the story as usual gives you a costume that makes it look like the second player is Sue. (Thanks, ContraNetwork!)
From the very start, people who begin a cooperative Cave Story+ game have an opportunity to choose a different look for the player two characters. The person playing as a second character has multiple color schemes immediately available. At the end of the video, at the 19 minute, 45 second mark, you can see how the Sue Costume is unlocked for the second player.
Cave Story+ is immediately available for the Nintendo Switch.