The short answer

In the browser and Steam versions, there is nothing to unlock. The character select screen has shown the full roster of 11 playable characters since 24 December 2013, according to the community wiki. You pick who you want and play.

Search interest in “unlocking all characters” is almost entirely a legacy of two other situations: the mobile app, and old web portals that shipped reduced demos.

Where locking actually happens

The mobile app

The mobile version is a different build. It uses a chapter structure with per-character level sets, and characters become available as you progress rather than being selectable from the start. A character’s unlock condition is tied to clearing levels with the characters you already have.

This is the version most people mean when they search for unlock methods.

Licensed demo builds on web portals

Demo builds licensed to browser portals historically shipped with a limited number of featured levels and a limited character selection. If you are playing on a portal rather than the creator’s own site, a missing character usually means you are in a demo, not that you have missed an unlock.

The level editor

Here is the useful loophole, and it applies to every version: all characters can be accessed in the level editor.

The wiki notes that the editor’s character set covers the full roster. So if a character is locked where you are playing, opening the editor and loading or building a level will normally let you play as them. This is the classic workaround and it is still the one worth knowing.

What to do, by build

You are playing What to do
Steam release Nothing. Pick from the full roster.
Browser version on the creator’s site Nothing. Pick from the full roster.
Browser demo on an external portal Expect a reduced set. Use the creator’s own site for the full roster.
Mobile app Progress through the chapters, or open the editor to preview characters.

Why “unlock all” community levels exist

Community-made levels named things like “unlock all characters” are short levels built to be finished in seconds. They were made for versions where progress gates content, so that players could satisfy the unlock conditions quickly.

Two cautions:

  1. They do not work in the Steam or browser versions, because there is nothing to unlock — they are just very short levels.
  2. Patch 1.99.1 was written specifically because of levels like these. It replaced the “Publish your first level” achievement with “Complete 10 Editor Playtests” to curb the flood of achievement-farming levels. If you are browsing the level browser and the top-rated lists look full of junk, that is why.

The optional “unlock” that is real: achievements

The Steam release adds a genuine progression system, and 11 of its 80 achievements are tied to characters — each requires beating three featured levels with a specific character. That is the closest thing the game has to character progression, and you can see the full list on the achievements page.