SNK vs. Capcom: SVC Chaos Cheats

Go to the character select screen and
highlight Kasumi and hold R

Alternately, you can access all the hidden characters except Red Demon and Young Athena on the character selection screen by holding R over the four top characters of the inner most rows of both sides. The following characters can be accessed by holding R while on the standard character's location at the selection screen:

Hold the R button over certain fighters:

Ryu unlocks Zero
Chun Li unlocks Demitai
Dhalsim unlocks Dan
Bison unlocks V. Ken
Vega unlocks Shin Akuma
Kyo unlocks Geese
Terry unlocks Mars People
Ryo unlocks Goenitz
Kim unlocks B. Iori
Katsumi unlocks E. Mr. Katate

Simply beat 35 fighters in survival to unlock Athena and beat 36 Fighters to unlock Red Arremer. Then hold the R button over Tessa to use Red Arremer and hold the R button over Genjyuro to use Athena.