Pico's Bizarre Adventure
Pico's Bizarre Adventure is a series of platformer games in the remix tree, using the same engine as untitled_279, and featuring Pico as he traverses a world parodying Scratch platformers. The Guardian of the Portal is also a recurring antagonist.
Guide
The first game is Pico's Bizarre Adventure 2, which requires no prior context. Pico's Bizarre Adventure 2.5 and Pico's Christmas Adventure 3 are also fairly standalone, simply building off of the first game.
Piconium features many of the same levels as the first three games, but adds extra levels at the end, as well as introducing the Guardian of the Portal. This pretty much means you can play through Piconium without missing much from the first three games – except that r/PicoPandemonium requires Piconium as well as the other games to understand. It's a direct sequel to both Pico's Christmas Adventure 3, and r/PaulaPursuit (the latter of which is part of its own series), and features elements from Piconium and Wicked Wicker.
Really r/PicoPandemonium is just a mess to try and follow. The best way to understand it is simply to play the four games before it (with 2.5 being the most skippable), as well as Wicked Wicker, and watching the whole Reddit series.