Created as SWN-001, as Bass he was a particularly dangerous Robot Master in the service of his creator, Dr. Albert Wily. He was made as a foil to Rock, and with a powerful Fortenium core, he had more power at his disposal than any other Robot Master on record. He initially posed as a false friend to Rock in order to fulfill his mission, and met him many more times on the battlefield afterward; one of the things that annoyed him so much at that point -- barring his failure to get a sound defeat over Mega Man -- was that Rock continued to have faith in him that he had potential to fight on the side of justice. An encounter with Duo reaffirmed this, the alien claiming he had 'justice energy' in him; a more interesting thing that occured during this period of time was the discovery that Bass could safety harness dark energy and use it.
As the years passed and the world changed, Bass and Rock twice worked together on the battlefield. The first of those times saw the rapid rise and fall of King; the second saw the coming of Rockman Shadow and his Dimensions, who were bent on eradicating all of the planet's life. While they worked together to destroy them, it took the aid of X to destroy them, and both Bass and Rock were nearly killed; only a conversion to an existence as a navi saved them. Bass was further damaged during the Alpha Revolt and escaped into the undernet, making a name for himself as Forte.EXE, consuming anything in his way. Dr. Cossack, however, managed to talk him into trusting him enough to return to his finally-repaired body, and over the time following that, he came to trust him as the father he never had in Wily.
As Bass came to adjust to the vastly changed times, he agreed to join the Maverick Hunters. It was, at first, simply for the sake of being back in the field of battle. Over time he came to see the massive injustices that all robots were facing, and in his off-duty hours would cheerfully go out and sabotage "retirement" facilities. There were of course others that he saved from the scrapyards directly, and the Hunters now count some of them in their roster; Bullet, for example.
And then came Innerpeace and Neo Arcadia. The entire mission, in his eyes, was sabotaged from the beginning. It took some time to see this, but the only reason he even participated was on a lark -- there was almost no official notice and it was misrepresented horribly. While in Neo Arcadia working in an undercover capacity, he underwent the humanoid conversion process. Due to his proximity to Mariah Fox as well as combat, he found that his flesh-and-body body retained most of the capacities it had while it was fully robotic... among them, the still-active weapon copy system and the ability to manipulate dark energy. Still, these changes forced him to alter how he fought and planned, and over the course of the months in Neo Arcadia, he found himself growing up and maturing. The brief relationship he unexpectedly found himself involved in only hastened that.
After Master Albert and Ouroboros were stopped from being a threat, and Master Albert himself was killed (twice), Bass returned and found that only a matter of weeks had passed, though subjectively he had experienced nearly a year. As a humanoid, he was considered legally human, and formally adopted by Mikhail Cossack, but chose to resign from the Hunters. A backup of himself was activated to function in his stead, without the memories and the burdens he had gained over that period of time, and the man who now bore the name Fortissimo Cossack left to find himself in the world. Perhaps older and wiser, but no less self-assured than he had ever been.
If only he'd had any idea how wrong things would eventually go thanks to that course of action...