Xen's human form is very similar to his Foxus from as shown by his silver hair and even his cape could be considered to look similar to his Tails.
With Xen being a part of the Foxus tribe, his rank is a very mysterious one. While being denied by the Elder from being the representative of the tribe, he wanted to prove his worth. He's one of the few Foxus that can use illusions instead of elemental skills. He left his home grounds in search of a nightmare he could outsmart and defeat.
Knowing the Foxus tribe was an enemy of Nemosku he headed to the Shadow Tower. Nemosku saw a glint of darkness in his heart and using his Elder's rejection to peak it, Xen decided to prove his worth in a different way, by taking down the legend the Elder belived in.
While trying to track down the new heroes, he meets up with the alien race the Collectors who are also after the heroes. After showing his progress he heads back to show his Elder how strong he truly is. Deeply dissapointed, the Elder banished him from their realm. Now truly angry and wanting revenge he vowed to truly make himself powerful and destroy the Elder's legend.
Xen unlike most Nightmares is very quiet and suprisingly gentle in certain situations. Before he joined with the nightmares, he was a kind-hearted Foxus that was interested deeply in the legends of the Legendary Heroes, wishing that he could one day meet them.
After joining with the Collectors he started to teach himself about technology, and after making his human illusion physical he started to carry a gun around the Collector King gave him. While Xen stays calm and collected at most times ever since the Elder rejected him he can snap after he doesn't get what he wants.
Since he was banished he didn't want to be a part of the Foxus tribe anymore, making him stay in his Human form. He only changes back into a Foxus if he truly needs his powers. Other than that he stays as a human and uses his gun to eliminate all things who interfear.
His own Collector men even fear him, when he snaps he kills anyone, friend or foe. Whoever gets in the way. He made himself a leader of all Nightmares, giving him infinite power. Enough power to take down even Nowtowla or Tabuu if given the chance.