Mobile Suit Gundam: Climax U.C.

Extra mode:
Successfully complete all of the missions for each main character in their series in Chronicle mode. This will unlock the final mission in Chronicle mode. Successfully complete that mission to unlock Extra mode.
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Crossbone Gundam X1, X2, and X3:
Successfully complete all the missions in each series of Chronicle mode. Then, complete mission 1 in Extra mode under the expert difficulty setting at least three times for all three Gundams.

Gundam F-90V:
Successfully complete all the missions in each series of Chronicle mode. Then, complete mission 3 in Extra mode under the expert difficulty setting.

Gundam F-91:
Successfully complete all the missions in each series of Chronicle mode. Then, complete the final mission on Chronicle mode.

Nu Gundam:
Successfully complete all the missions in each series of Chronicle mode. Then, complete mission 2 in Extra mode under the medium difficulty setting.

Sazabi:
Successfully complete all the missions in each series of Chronicle mode. Then, complete mission 2 in Extra mode under the expert difficulty setting.

Z Zaku:
Successfully complete all the missions in each series of Chronicle mode. Then, complete mission 1 in Extra mode under the beginner difficulty setting.

ZZ Gundam:
Successfully complete all the missions in each series of Chronicle mode. Then, complete the final mission in Chronicle mode.