After you beat the game, play through and beat Survival mode. Afterwards, a message says "A New Party Member has been added." To use Achilles, go to Survival Mode, and choose preset party. He will be at the bottom.
The following modes can be accessed on the Title Screen under an additional "Extra" menu option.
|Unlockable||How to Unlock|
|Collection||Beat the main game|
|New Game+||Beat the main game|
|Records||Beat the main game|
|Survival||Beat the main game|