Duon

Duon is the sixth boss of The Subspace Emissary in Super Smash Bros. Brawl. It is a two-sided robot; 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. Duon moves around on one set of wheels, both of which depict the Yin-Yang, a symbol that represents balance. Both sides have shoulder exhaust pipes, which like the arm cannons, can fire rockets and mines from the pink side and blue side respectively.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Duon is located atop the Battleship Halberd Bridge. It is fought by Fox, Falco, Snake, Lucario, Zelda/Sheik, and Peach.

Six Mr. Game & Watches are launched through the glass of the Halberd's control room and land a few feet away from where Zelda, Peach, and Fox stand. They dissolve into Shadow Bugs and quickly reform into Duon, which Fox, Zelda, and Peach prepare to battle. Lucario and Snake jump from the control room's window and land behind the three. Falco lands behind the fighters after ejecting from an Arwing as it flies overhead. All the characters work together to defeat Duon. After being defeated, Duon shrinks, and loses all of the shadow bugs that made it so powerful. A lone Mr. Game and Watch trophy lays in its place. With the Mr. Game & Watches gone from the wheel, Meta Knight regains control of the Halberd. Duon later appears in The Great Maze, where it appears at the same area, atop the Halberd's bridge.

Its attacks involve spinning rapidly, firing a barrage of missiles, crushing the player by jumping, and shooting lasers in a circular pattern. Duon often rotates to change the side facing the player. While Duon fires the missiles, the player can move behind Duon. The missiles may hit Duon, dealing damage to it.

Super Smash Bros. Ultimate
Duon also appears in Super Smash Bros. Ultimate as an Advanced-class Attack-type primary spirit with two support slots. The spirit can be encountered in the Base sub-world of the Light Realm in World of Light. The spirit's puppet fighters are giant Ice Climbers on the Halberd stage, referencing Duon's battle from Super Smash Bros. Brawl.

Trivia

 * In the trophy gallery of Super Smash Bros. Brawl, Duon's name can be spotted on the underside of its trophy.