Doppleganger
[PS2 Cheat by: Cheat Code Master ]
A secret boss that gives up the key item by the name of, "Own King." This item is worthless in Vol. 1, but you can get level 100 weapons in the next two versions.

Vol. 2 preview
Once on the main menu, hold R1 + R2 and quickly press Square, Triangle, Square, Triangle. Release R1 + R2, then hold L1 + L2 and quickly press Right, Up, Right, Up. Release L1 + L2, then quickly press L3 + R3. If you entered the code correctly, the Vol. 2 preview will be available in the media performer.

Bonus emails
Once on the main menu, select the "Convert" option. Convert the saved game files from .hack//Infection, .hack//Mutation, or .hack//Outbreak to get an email from BlackRose that was sent to Kite after .hack//Quarantine. Convert a saved game file from .hack//Quarantine to also get that email in addition to an email from Hokuto sent to Rena. *** In the Japanese version of the game, convert a saved game file from .hack//Fragment to unlock the email from Hokuto.

Alternate Tri-Edge
Convert a saved game file from one of the previous . hack games. When you fight Tri-Edge at the end, he will have the same name as your character from the previous .hack game.

PS2 Cheats for .hack G.U. Vol. 1: Rebirth

.hack G.U. Vol. 1: Rebirth cheat codes

.hack G.U. Vol. 1: Rebirth cheats