Game Boy Advance Game Link cable

The Game Boy Advance Game Link cable is an accessory for the Game Boy Advance that enables connection between two handheld GBAs, for the purposes of multiplayer gaming, cooperative play, or unlocking hidden features. The cable can also connect between two Game Boy Advance SP systems, or two Nintendo GameCubes (requires the Game Boy Player).

Metroid: Zero Mission has an unlockable feature that requires this cable. If a Game Boy Advance system with Zero Mission is connected with another that has Metroid Fusion in it, the ending images from Fusion can be unlocked in Zero Mission. This includes endings exclusive to the Japanese version of the game, which depict events from Samus Aran's childhood. The game link is initiated from the Options menu at any time, even without ever completing Zero Mission.

Manual
"If you link Metroid: Zero Mission to Metroid Fusion, an added bonus will appear on your Metroid: Zero Mission Game Pak. After loading data from Metroid Fusion, an option to view the Metroid Fusion gallery will appear on the Options screen. View these images just as you would the Metroid: Zero Mission images.

Loading Data from Metroid Fusion ''Follow the instructions for linking your systems on page 39. On player one's Game Boy Advance, choose "Metroid Fusion Link" on the Options screen and press the A Button to begin loading Metroid Fusion data.''

''Note: If the link fails, an error screen will appear. In this case, check all cable connections and try again from the beginning.''