Megaman was introduced in Megaman 7 as an anti-villain. So at first he was introduced to counter Megaman with his dog Treble, an anti-thesis of Megaman and Rush. So he seemed to be a hero at first but in reality, Dr. Wily created him to counter Megaman and perhaps to succeed Dr. Wily. But unlike his master, Bass was more inclined to simply only focus on Megaman rather than the typical take over the world making him more of an anti-hero later. What I like about him as a villain is that unlike Dr. Wily, he really is not afraid of Megaman while later, he becomes more of a anti-hero.
During Megaman 8, he was seen again fighting Megaman in the opening scene. During that game, he is only focused on fighting Megaman than becoming Dr. Wily's successor. In the Power Battle series, he is seen siding with Megaman for his own purposes. In Megaman 8, he did temporarily have the "evil energy" to which he thought he could defeat Megaman. What was weird is that he never went berserk, he is more or less who he is, that douchebag who rivals Megaman and opposes his creator Dr. Wily.
So he did appear again in Megaman and Bass which was released in Japan as Rockman and Forte (Forte is his Japanese name). In that game you can either play as Megaman or Bass which was later done in X4 where you can play as either Megaman X or Zero. In that game, he doesn't have a charged shot but he can fire rapid bullets at any direction but he can't run and fire which sucks at that point. In Megaman 10, he's more of an anti-hero than a real villain. He's not working for Dr. Wily anymore but he is still bent to proving himself better than Megaman.
So why am I bringing Axl here? I thought Axl had some similarities to Bass. Both of them are anti-heroes who use rapid fire bullets. So perhaps Bass behind the scenes got remodeled into Axl but this is only my speculation or Axl could be based on Bass. Axl is sort of a rival to Megaman X but unlike Bass, he is dedicated to fighting the bad guys. Also like Bass, Axl is introduced in the 7th game of the series but this time as playable character and not just Megaman's rival. Unlike Bass, he can hover but can't double jump and he can actually mimic some enemies, in Megaman X Command Mission he can do a super move where he mimics either which bosses when he is at 100%.