Big Fau was first seen in Roger's memories of "The Event" in "R.D." and made its first full appearance in “The Third Big”. It was called "The Third Big" and the "White Big" by Vera Ronstadt, and Alex Rosewater believed it to be the most powerful of the "Big" Megadeus.
The parts for Big Fau were transported within the three Foreign Megadeus that fought Big O offshore of Paradigm City in "R.D." and "Roger the Wanderer". After they were defeated, their remains were recovered by the Paradigm City science division and the relevant parts re-assembled. The Megadeus, however, lacked a memory core. Ultimately, this vital component was disregarded, and Big Fau was activated and piloted by Rosewater.
During the first battle against the Union Megadeus Bonaparte, Alex controlled the Big, allowing him to destroy Bonaparte with ease. Once Bonaparte was vanquished, however, Big Fau's dorsal beams disobeyed Alex's orders and began to attack 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 jury-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 killed Alan Gabriel with the message "Ye Guilty". Big Fau was eventually disabled by Big O's Final Stage at the end of the "The Show Must Go On" and removed by Big Venus.
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 water support for the Big O land units and Big Duo air units.
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.
The primary weapons are the turbines mounted just below its hands, known as "Saw Cuffs". 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 include 2 heavy laser cannons mounted in its shoulders and laser projectors that extend 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 the warheads. Additionally, Big Fau is equipped with an energy shield which can repel Big O's piston attacks, along with other missiles and lasers.