I consider the game fully done with this update.
Don’t expect any major updates from now on. :3
And this took a lot longer to release than I expected

Changelog:

• Added OS X port (please report any anomalies)

• Added Endless Mode and online leaderboards for it.

• Smoothed out camera when moving over the edge of a level or using stairs

• Several minor, unnoticeable changes in the code (Don’t take my game code apart. Please.)

• No more wonky collision behavior like this (I think)

• When you are in a 1-tile-high gap, you can no longer stand up

• New achievements: It’s over 1k and Master Collector

• Added a new character, linked to that achievement

• Changed the save data format; old save data will automatically be converted upon the first startup

• The reference file for custom characters has been updated

• Removed unnecessary libraries from game code

• The player list will no longer be updated every time you open the character selection

• “Toaster-Check”: If the game is running below 50FPS (usually 60FPS), the game will slowly reduce graphics settings to try to improve performance

• Music is now loaded in a separate game thread

• Replaced ‘nothreadedsaving’ argument with ‘singlethread’ argument; GJ-Thread is excluded from this argument

• Removed a possibly game-breaking password… Could definitely crash the game.

• Probably something else I forgot about…

v1.1.1:

• Player selection now properly updates when a new character is unlocked

Also:
Now that the game is finished, I decided to hold a little contest…
The first person to get every achievement (or to meet their requirements) in 1 run of the game and send me a video recording (screen recorded, not with a camera) of that, will get the chance to have ANY message they desire in the game, in any place they want to have it (limited to 511 bytes; no inclusion in existing cut-scenes; I reserve myself the right to censor any part of it.).
Preferably via YouTube link on Twitter (@DPlayer234 ), not in the game’s comment section.
Extra points for creativity. Go ham and share the challenge…