SNK vs. Capcom

Fight as Dan:
Hold Start, then press Down, Left(7) at the character selection screen.

Fight as Demitri:
Hold Start, then press Right(3), Left, Up, Right, Down, Up at the character selection screen.

Fight as Evil Ken:
Hold Start, then press Up(2), Down, Left(2), Right, Left, Up at the character selection screen.

Fight as Geese:
Hold Start, then press Left(3), Down, Left, Up, Right, Up at the character selection screen.

Fight as Goenitz:
Hold Start, then press Down(3), Right, Down, Left(3) at the character selection screen.

Fight as Mars People:
Hold Start, then press Down, Right(3), Up, Down, Left(2), Down at the character selection screen.

Fight as Riot Of Blood Iori:
Hold Start, then press Up(3), Down, Left, Right, Down, Left at the character selection screen.

Fight as Zero:
Hold Start, then press Up, Right(2), Left(2), Down, Up(3), Right at the character selection screen.

Random character selection:
Hold Start at the character selection screen.