Beat Extra Mode as every available jet. Doing so will unlock that jet's alternate make for every mode of game-play. Press DOWN during the Select Fighter screen to access the secret jets.

Unlockable	How to Unlock
Proto SIF	Beat Extra mode using the SIF
Seylen-II	Beat Extra mode using the Seylen-EX
Silph-Assault Jet	Beat Extra mode using the Silph-II
Valkyrie-II	Beat Extra mode using the Mega Valkyrie