X-Men: Mutant Academy is a lousy Gameboy Color fighting game that features a 2-player link cable head-to-head mode.
There are two modes available. VS is the standard 1-v-1 match whereas Battle is a 3-on-3 fight only without tagging.
Two secret fighters can also be unlocked by entering their respective codes on the main menu but they probably should have been unlocked by default. Also, the link cable needs to be inserted before the GBC systems are turned on to enable the multiplayer mode.
