Some basic notes about my portrayal - some are briefly mentioned on my Hades/Emet-blog already - since I do not know when I have the time to write them in full detail.
Hythlodaeus possesses the ‘sight’ to see the souls and the underworld, albeit such trait is not that uncommon particularly among their elder. However, similar to Hades (later: Emet-Selch), his talent is greater than for most of their kin. He is one of the few who can recognize the different hues in the souls and can read, although never truly specific, their mood.
Hades, who is one of the oldest ancients, has been, in fact, his mentor when he was young. He taught him to use his uncommon gift wisely; which then also aided in his promotion to the Chief of the Bureau the Architect.
Albeit they may have started off as unlikely pair of stoic mentor and enthusiastic student, Hythlodaeus and Hades became very close over the eons - viewing each other as their best friends in the very end.
Hythlodaeus is also one of very few ancients who indeed were aware of Hades full potential; which is also why he declined the position of Emet-Selch, knowing that he was unfitting for it when his friend was not only more gifted but also far wiser.
Hythlodaeus was born around the same time as the later Azem ( @starled ), thus being close friends with her as well. In fact, for some centuries he had some sort of romantic interest in her until he realized that Hades also harbored feelings for her, ultimately stepping down for him.
Since his personality is rather outgoing and empathetic, Hyth has made all kind of friends in and outside of Amaurot. He also takes a liking to gossip, albeit he never ever used this for his own advantage or to damage someone’s reputation.
Hythlodaeus is very gifted in creation magicks, however his own imagination is quite lacking. He has trouble to think of the simplest concepts by himself, yet manages to create the most magnificent creations based on (very detailed) concepts of his brethren.
His silhouette is quite tall for their kin, and yet his friend Hades still towers over him. His hair is shortish ravenblack, albeit this very fact is scarcely ever visible beneath his cowl.