This simple Ultrakill mod will add the Mystery Man from Undertale to almost every level of the game as a SECRET. Them being in the hardest places to find on the level, even requiring for going out of bounds with glitches for some.
Just go to the campain and try to find one in every level listed in the find list. To see the find list, just pause the game and the list of every gaster and level should appear. White gasters mean that you have collected them, transparent ones mean that you haven't. Blue gasters mean that they are allowed to be collected with cheats on.
Make sure that you have an INTERNET CONNECTION, as the mod will download certain files from duviz.xyz.
Not really, in fact, if you have Angry level loader on, there will be an extra gaster, even though compatibility is still being worked on.