Big Fau's primary weapons were the turbines mounted just below its hands. When activated, the turbines could grind through practically any material or even allow Big Fau to drill its fist straight through an opponent's body. The fists themselves could also spin, giving the megadeus' punches an incredible amount of destructive power. Additional weapons included two heavy beam cannons mounted in its shoulders and laser projectors that extended on flexible periscope-like extensions from its back. Big Fau was also able to fire its arms at an opponent, the turbines drilling through any obstacle before detonating. Big Fau is equipped with some kind of energy shield which can repel Big O's fists and other matter-based attacks (missiles, bullets etc.). Judging by Big Fau's streamlined appearance, its turbine wrists, its ability to travel effortlessly through the water, its conning tower shaped head as well as a conning tower on its back, and the flexible periscope-like laser cannons on its back, it is likely that Big Fau units were used primarily for naval combat, functioning as heavily armed amphibious craft, and providing ocean-based combat support for land- and air-based megadeuses.
During its maiden battle against the Union Megadeus Bonaparte, Alex could completely control the Big, allowing him to tear Bonaparte to pieces before finally destroying it with Fau's dorsal beams. Once Bonaparte was vanquished, however, Big Fau's dorsal beams disobeyed Alex's orders and began to assault the domes. In response to Alex's claim that he was Big Fau's dominus, Big Fau displayed the message "Cast in the Name of God... Ye Not-" before shutting down. Later, Big Fau was jerry-rigged into working by Jason Beck using R. Dorothy's Memory Core, finally completing Fau and allowing it to declare Rosewater "Not Guilty." Fau is also the only known Megadeus to fuse with its pilot, Rosewater, via a series of cables plugged into its pilot's back, although Big O attempted to do the same with Roger, and Big Duo wrapped, and subsequently mummified, Alan Gabriel before rejecting him with the message "Ye Guilty". Big Fau was eventually disabled by Big O's Final Stage at the end of the Act:26 and removed by Big Venus. Big Fau may be a reference to Frankenstein's Monster because of its long head and bolt-like ears, as well as the nature of its reconstruction.