In which game does Proto Man first appear as a boss?

Proto Man first appears as a boss-like enemy in Mega Man 3, where he operates under the name Break Man. Mega Man encounters him several times as a masked mini-boss during the game’s stages.

Mega Man 3 was released by Capcom for the Nintendo Entertainment System in 1990. Its story presents Break Man as a mysterious rival whose motives are unclear: he fights Mega Man, but his actions do not fit neatly into the role of a conventional villain. After being defeated, he often opens a passage that allows Mega Man to continue.

The character’s identity is revealed near the end of the game, when Dr. Light explains that Break Man is actually Proto Man, Mega Man’s older brother and the prototype robot created before him. This is why some references describe his first appearance as Proto Man in Mega Man 3, while others emphasize the Break Man name and his mini-boss status. The game also introduced Proto Man’s signature scarf, shield, and whistle.