Bass is Megaman's rival starting with Megaman 7 and so on. Created by Dr. Wily he serves as a kinda Dark-Megaman though he's not terribly evil and sometimes lends a hand to Megaman.
I wish he'd come out sooner, because 7 was a bit late in the game.
Since I never liked 7, I didn't play enough to see any of the cooler parts.
Megaman might have a helpful robot dog named Rush, but Bass has one-uped him. He has a fierce robot dog named Treble.
He also has some pretty dorky back-story.
Bass and Megaman even starred in Megaman & Bass originally on the SNES (Japan-only) and an American release on GBA.
You could unlock him in Megaman 10 for WiiWare, PSN, and XBLA. He was a decent alternate character.
Bass is named after the musical instrument of the same name. In Japanese his name is フォルテ (Forute) which is based on Forte a dynamic in music.