Mega Man & Bass
Mega Man & Bass is part of the classic Mega Man series, where you can control either Mega Man or Bass, as Dr. Wily and Dr. Light have made a truce to fight King. The game was originally released only in Japan, but a later Game Boy Advance port had an international release.
Story
After Mega Man 8, Dr. Wily's whereabouts were still unknown, and he built a new castle to prepare for his comeback. But then a robot called King appeared and planned to conquer the Earth. He believed that robots were not only there to serve humans, but that they were superior and therefore should rule the Earth. King ousted Dr. Wily from his own new castle and stole the data from the robots created by Dr. Wily and Dr. Light to build his own robot army. To stop King, Dr. Wily and Light made a truce and sent Mega Man and Bass to stop King's rebellion. When King attacked a robot museum to collect data for his army, Proto Man tried to stop him, but King cut him in half with his axe and sent him to Dr. Light's lab for repairs. Mega Man/Bass confronted King, but he backed off claiming he didn't have time for them, leaving the Green Devil to take care of them. Mega Man and Bass set out to foil King's plans.