Swimsuit mode
Win the game once to unlock Swimsuit mode in the Memory Code event viewer.

Clothing
Unlockable How to Unlock
Ziggy Swimsuit Clear Advanced Hakox Levels.
chaos Flawless Beat game, save "Clear Data," load that data.
KOS-MOS D Unit V1 Buy it at a store.
Shion White Shirt Steal from Red Testament.
Shion Swimsuit Pedea Island, West Side: EVS to this island and search
Shion's housing for a treasure box.
MOMO Swimsuit Dabrye Mine's Entrance: Defeat Mai, return to entrance, play with the crane game a few times.
KOS-MOS Swimsuit Elsa, E.S. Bay: Complete Segment Address, access this data to confirm completion, chat to Theodore.
Jr. Swimsuit Miltia World Map, Right Side: Open Treaure box.
Jin Swimsuit Labyrinthos, Margulis's Room: Treasure box.
chaos Swimsuit Floating Landmass Crypt through EVS: Use console at the entrance to the temple to gain access to a secret area.
Allen Swimsuit Beat game, save "Clear Data," load that data.
Shion Research Uniform Steal from Red Testament's ally.

Easter EggsShions "Vector" uniform
Have a Xenosaga episode II clear game data save on your memory card, And at the Title Screen of Xenosaga episode III, start a new game. And it lets you load up a Xenosaga episode II save file. Once you do that, Shion will have her "Vector" costume/outfit, for the new game.