Basically you get a bunch of fighters together (12 in this case) and get them to punch, kick, bite and thump the bejesus out of each other then you're close.
In Strider 2, the player takes control of modern-day ninja named Strider Hiryu who wields a deadly plasma-ejecting cypher named the Falchion, completing missions assigned to him.
Video game sequels are a funny lot, while many succeed on expanding franchises and introducing new elements to already established worlds, characters, etc.
It's hard to say just why 2D fighting crossover games exist. Well, actually, I think I know why they exist (hint: it starts with a $ and ends with another $).