Relationships
Kevin Cecil (Uriel)
They are very close. William is very grateful for Uriel, and cares for him deeply, enough to forgive him the deception of posing as his friend Kevin (remembering all the time they spent together and how Uriel took care of him). He credits Uriel as the reason he was able to live freely up till then.
William is implied to be one of a very few that can scold Uriel, and Uriel is likely to carry out any of William's requests (he even helped demon's on his request). Uriel has busted into Hell to protect William (the only angel to have done so, ever). Uriel care for William opinion on him, and his well being.
Dantalion
Dantalion first meets William when the latter accidentally summons his pentagram located at William's basement. He immediately calls William the "Elector", the one being who possesses the power to elect the next Interim ruler of Hell in the absence of Lucifer. Being a realist, William dismisses Dantalion's words.
They often argue with each other, mostly because Dantalion keeps on insisting William to vote for him, and William, claiming that he will not be choosing anyone. Despite their initial dislike, Dantalion and William gradually grew closer, with Dantalion even promising to protect William with his life, and William, starting to worry for Dantalion's safety.
Sytry
Sitri and William's relationship is a little better than William's and Dantalian's. While William does get tired of Sitri telling him to chose Sitri to be the substitute king of Hell, he has shown no obvious evidence of ever hating him, and even seems sad when he learns that he would never see him again if Sitri was to go to sleep. (Though he did this with Dantalian and Camio too.)
Camio
Camio and William are close friends. Before he knew Camio was a demon, William looked up to him as Nathan Caxton. Camio, like Sytry and Dantalion, is protective of William. Unlike Dantalion and Sytry, not even once Camio asked to be elected. It is implied before his identity as a demon was found out, Camio had no interest to be interim ruler even though Beelzebub has put his name forward. It was noted by Dantalion that Camio doesn't seem to be particularly interested to convince William to elect him, and most likely stepped in to the election to have a legitimate reason to protect him (it is later revealed that he has very much become interested in being the Emperor himself, but has different methods to go about achieving it).
Camio has told Lucifer that he wishes to protect William and William's way of life (as it is what Solomon longed for).[1]
Isaac Morton
Willam's closest friend in school. Despite the obvious differences in their personalities, William and Isaac get along well.
Solomon
William's ancestor (and previous incarnation). William does not show much interest in Solomon,and the same can be said for the latter as Solomon did not think or show to much thought about William when possessing his body.
William considers Solomon annoying, and "won't let him interfere as long as he lives", and Solomon seems to think of William as interesting[2].
Kevin Cecil (Human)
Kevin and William are childhood playmates, but have drifted apart after the death of William's parents and with Kevin's studies in college (as well as Uriel impersonating him). Kevin used to indulge William, even going as far as eating rocks and caterpillars William fed him as a child.
References