Access Katze:
If you beat Slash mode without dying once you can access Katze 
(The gun toting mini-boss) in Versus mode.

Get to the bosses:
In order to skip all of the normal characters and go straight to the 
sub bosses you must: First, cripple your first opponent so that he/she 
CAN NOT walk. Then run to the manhole that you would normally jump 
down after defeating the last normal opponent. By doing this you should 
be able to skip through about 3 or 4 fights.

Note: Rumor has it that if you do the above and beat all the bosses 
without continuing, you get to see your character's secret ending!!