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 has a curved blade on his 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). He rotates 180 degrees to change his attack strategy. The wheels he pivots on are the yin and yang symbols from Chinese belief, symbolizing balance and juxtaposition of two forces.
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. He is fought by Snake, Lucario, Fox, Sheik, Peach, and Falco. It is basically a complete stage, Halberd Bridge. After he is defeated, he dissolves into Shadow Bugs, leaving behind a trophy of Mr. Game & Watch. Duon is later rebuilt by Tabuu to face the heroes in the Great Maze.
The time has come, it must be decided. Tabuu has created two massive generals, both robots, both tough. He knows that one will have the important job of protecting the halberd and one must be a general on the ground, to be deployed to make sure no rebellion is to get far. He knows that he must find out which of these two creations is the more powerful and he knows there is only one way to find out. Slowly the master hand begins to transmit a signal to gannondorf, the orange hooks holding it back pulsating.
3 days Later
The two levitan robots ready for the fight ahead, knowing full well what the differance between victory and defeat. The recently completed collesium is jam packed with Primid's, each one cheering for ethier Duon, Galleom or both. The gates slowly roll down and galleom is the first to exit, charging forward in tank form. Duon switches round to it's pink side and fires a cluster of missiles, which weave and swerve around, crashing harmlessly in front of galleom, who carries on charging. Duon wheels out of the way of the attack and sends a barrge of lasers, most missing their target due to the method of deployment, with only one striking galleom. Galleom swings round and attempts to slam into duon, but it gracefully wheels out of the way.
Noticing it's plan is not working Galleom begins the slow process of transforming back into robot form which allows duon to get a few cheap shots with it's missiles. However galleom is unfazed by this and sends a few missiles of his own back at the duel sided shadow bug boss, who is oddly enough barely effected by the missiles slamming into him. Galleom then charges and delives a massive uppercut, but duon wheels around it, switching around to it's blue side as it does. It swings it's head around and slams the axe head into galleom's side, who flinches in pain. Galleom grabs him by the head and spins him away, dazing duon and allowing for him strike him smash it with a massive punch. Duon shakes it's head furiusly before spurting forwards and delivering a series a quick jabs, which catch galleom of guard and force it too it's knees. Duon then leaps into the air and slams it's axe head into galleom's chest, crushing the outer armour with ease and leaving it convulsing in bursts of electricity. Tabuu (who has been watching from inside subspace) raises his hand, calling off duon's attack and making it return to it's waiting place. Tabuu smirks an evil grin, finally knowing which of his creations will defend the halberd.
Duon won because of two main reasons. The first was becuase of it's mobility, it was easily able to dodge and weave around galleom's attacks. The second was that unlike galleom who was a very close range orintated fighter, Duon was desgined to be able to fight at both long ranges and close quaters combat.