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 demons on his request). Uriel has busted into Hell to protect William (the only angel to have done so, ever). Uriel also cares for William opinion on him, and his well being.
Dantalion
Dantalion happy to see William again.
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 right away and thinks he is only a charlatan.
They often argue with each other, mostly because Dantalion keeps on insisting William to vote for him, whith William claiming that he will never chose anyone. Despite their initial dislike for each other, Dantalion and William gradually grew closer, with Dantalion even promising to protect him with his life, and William starting to worry for Dantalion's safety, fearing he may get killed. In Pillar 95, it's stated that Dantalion has developed romantic feelings for him.[1] William's own feelings for Dantalion might also be of love, although this is not yet confirmed.
Sytry
It is hinted that Sitry and William's relationship is a little better than William's and Dantalion's. At the start of the series, Sytry was only interested in William because he was the elector. However, as the story progresses Sytry becomes closer to William, as shown when he left the demon party to save him from Baalberith's forces. Sytry becomes very protective of William after finding out that Dantalion had killed Solomon.
While William does get tired of Sytry telling him to chose Sytry 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 Sytry was to go to sleep. (Though he did this with Dantalion 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).[2]
Astaroth
Astaroth holds some interest in William . She has a bittersweet expression on her face when she mentions feeling nostalgia at their meeting , but being the only one that felt it.
She had no qualms in trying to force William into choosing Dantalion as the Substitute King. And has repeatedly expressed concern in protecting William from Heaven, which sought to make him ascend and turn in into their puppet.
Gilles de Rais
Gilles for the most part doesn't care about William. He does, however, seem interested in Solomon, and as his current vessel, William has Gilles's attention. Though most of his attention is beacuse of his leader, Beezlebub.
Michael
Michael is only interested in converting William into being one of Heaven's pawns because he is the reincarnation of Solomon. William sees Michael as a threat and an enemy.
Metatron
William and Metatron have not been shown interacting that much, though he did become William's "fag" and spent time with him, their relationship could be comparable to Isaac and William's friendship. Metatron, as Sean (or Sion), would perform menial tasks for William as his "fag" such as collecting club invitations for him. He would also join William for tea time. When Kevin reveals Sean's true identity to him, William displays a disconcerting expression, but says nothing.
Isaac Morton
Willam's closest friend in school. Despite the obvious differences in their personalities, William and Isaac get along well, even though Isaac annoys William sometimes. Isaac is also an exceedingly important person to William (demonstrated when Michael inquired as to what William felt was most important to him).
Solomon
Solomon and William meet with each other face-to-face
Solomon and William have a very complicated relationship. William does not show much interest in Solomon ,and the same can be said for the latter as Solomon did not think nor give 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[3].
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