Hold the R button over certain fighters to unlock:

• 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.