Its name is derived from the Latin word duo, a word meaning twosome, or a group of two, alluding to its two-faced design.
Duon is a two-sided robot on one set of wheels; one half is pink and has a head cannon and two arm cannons, and the other half is blue with two arm swords and a curved blade on its head. Both sides have shoulder exhaust pipes which rockets (pink side) or mines (blue side) can be fired from (though these can also be fired from the arm cannons). It rotates 180 degrees to change its attack strategy. The wheels it pivots on are the Yin and Yang symbols from Chinese belief, symbolizing balance and juxtaposition of two forces. There is an invisible platform between Duon's two halves, making Duon one of only two bosses that a character can actually stand on, the other being Porky Minch.
Role in The Subspace Emissary
After Snake, Lucario, and Meta Knight kick all the Mr. Game & Watches out of the Halberd's control room, they fall to the bridge and dissolve into Shadow Bugs, which form into Duon, a last resort to defend the Halberd. It is confronted by Snake, Lucario, Fox, Zelda/Sheik, Peach, and Falco (Meta Knight stays behind to retake his ship and does not take part in the fight). The boss battle makes up the entirety of the Battleship Halberd Bridge level. After Duon is defeated, it dissolves into Shadow Bugs and leaves a trophy behind of Mr. Game & Watch.
Duon later reappears in The Great Maze and is refought by the playable characters.
Like all bosses Duon is controllable with hacks in Boss Battles Mode. The start button self-destructs any boss.
HP and damage taken
Like all bosses in Brawl, Duon gains an additional ×0.6 damage resistance modifier when fought in co-op mode, effectively gaining 67% more health. This modifier is applied whenever the second player is in-game, and is no longer applied if the second player loses all their stocks.
Duon's trophy is obtained by using a Trophy Stand on it during the boss fight.