When selecting a song, press Option (instead of A button).
Instead of the normal symbols scrolling on the screen (A, B, Arrows), it will instead be Fishes, Honey Jars (or something like it), and Mice (respectively). Also, Felicia will be in her other costume.
On the Character Select Screen, Press and hold A on the character you select to get a new costume color.
To access blood in the game, simply change the language to Japanese in the internal settings of the NGPC.
Successfully complete game on Gamer difficulty.
After choosing either Tag or Team mode, select one character (or even two, if playing in Team mode). Afterwards, press the option button, and you'll play down a man (or two).
There are eight hidden characters. In order to play as them, you must beat the game and at the end, a picture of one of the characters is shown covered with blocks. It will randomly remove a few blocks, the higher the difficulty the more blocks. When all are uncovered you get that character.
NOTE-this is completly random.
The characters are Yuri, Akuma, Akari, B.B. Hood, Geese, M.Bison, Orochi Iori, and Evil Ryu.
Press Option right as you see the SNK Vs. Capcom logo (It may take a couple of tries). You will be able to select any of the ingame cutscenes with any character or team.
To play as another character from the Metal Slug series in the mini-game Target, simply press Option before the ''READY'' sign comes up.