Crossover

The so called bonus/secret/whatever characters seen in vs fighting games should not count for this tag as they're more like easter eggs than actual crossovers. For example, Darth Vader found in Soul Calibur 4 is not eligible.

Basically, characters that would never be placed together by any logical means other than divine intervention (aka: the marketing geniuses)

This is not about characters or even sets of characters making a cameo. The crossover is not just about scenery. Each 'faction' must contribute an equal or at least significant portion of the story. For instance, the crew of the USS Enterprise 1701-D appearing in Ultima VI: The Black Gate does not count since the vast majority of the game is all about the Ultima universe and characters.

It does not have to be two 'factions'. It can be 3 or more.

This tag can also apply when two or more character sets by the same company are mixed. But they must be significantly unrelated to begin with. For instance Mario and Zelda are not related. Mixing Mario and Zelda characters would be eligible. Mario is connected to Donkey Kong and also connected to Bowser, so mixing any combination of Donkey Kong, Bowser, and/or Mario does not count.