Duo was our overlarge final boss from EXE / BN4, and his role doesn't differ much in his anime counterpart. Duo has taken it upon himself to place judgement upon the Earth: is it worthy to survive? The mass destruction caused by Dr. Regal triggered Duo's attention from his asteroid home deep in space. Dismayed by the outcry of corrupt evil, Duo begins the process of "Earth Erasure".

Of course, Net Saviors Netto and Enzan could not just stand by and watch their home be destroyed. Duo becomes interested in their ability of Cross Fusion, this special bond between a human and its navi. And so, Duo decides to temporarily spare the world its demise, in order to give the "chosen ones" time to purge the world of its evils.

Duo's subordinate, Slur, makes the trial harder for those who are picked for such a journey. She randomly picks those whose hearts of full of darkness, and exploits their desires to the point of causing havoc. Netto is the first to be marked by the "Crest of Duo", and later Enzan. It is up to these marked individuals to prove to Duo that not all is yet lost in their world.

It may not seem like it, but Duo actually has legs. XD He's one HUGE navi...thing....whatever he is.
This character description was made for episodes that I have seen so far. It may be updated as more episodes become available.