Alien

Alien (エイリアン) is the final boss from Mega Man 2. When the player enters in his room, Dr. Wily will jump from his flying saucer and will turn into a green alien. His only weakness is the Bubble Lead, and all other weapons refill all his energy when used against him. He shoots and flies slowly through the screen, always flying in ∞ form. When it is defeated, it is revealed that he was just a hologram created by Dr. Wily, and the alien is a small flying ball.

Trivia

 * In Mega Man Network Transmission, the Life Virus' embryo-like second form acts almost exactly like the Alien.
 * Mega Man 2's subtitle "Dr. Wily's Secret" refers to the scene in the game where Dr. Wily turns into the Alien.
 * The reason the Alien is only weak to Bubble Lead is that it is only a hologram- the bubbles do not actually harm the Alien, but instead cause the projector to short-circuit
 * The Alien is the only final boss in the entire Mega Man series that is immune to all weapons except one (Bubble Lead)