And yes, Tempest is right. Akira, Pai and Sarah all have a "Great" winpose. Simply start up VS mode with shortest life and 1 round, win without taking damage, then release all buttons after you see "K.O." appear on the screen. You should get it.
I know that certain win poses are triggered by holding a button combination after you win the match. For example, after ending the match with a GREAT with Jacky you get the win pose: ''That wasn't your best shot.'' But you can also manually trigger the win pose with H+K, regardless of whether you finished the winning round with a GREAT or not.
It works the same way with other characters. You can trigger GREAT win poses with other characters as well while holding H+K after you've won the match:
''That was absolutely awful.'' - Christie
''Hmm... the face of death.'' - Leon
''You should really put some ice on that.'' - Kokoro
''Better luck next time, huh?'' - Momiji
''Even tougher, even stronger.'' - Tina