I remember when Proto Man debuted in Mega Man 3. He would show up in the middle of a stage and fight you, then disappear. Since his helmet was on the title screen, his origins and motivations were quite mysterious. Though he seemed like a foe, he eventually ended up helping Mega Man. Way later in the series, he became playable; Proto Man mode is DLC for Mega Man 9, and Proto Man can be used from the start in Mega Man 10.