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 his safety. In Pillar 95, it's stated that he has romantic feelings for William. William's own feelings for Dantalion might also be of love, although this is not confirmed.
Dantalian and Sytry have a fierce rivalry. It appears that even before they were shown in the manga that they knew each other but did not get along. Their relationship is futher complicated when Sytry finds out about Solomon's murder. Sytry didn't care when Dantalian went missing, but was unnerved when he returned.
Although having very few interactions with Dantalion, Camio shows respect for Dantalion in spite of his Nephilim status. Dantalion in turn acknowledge Camio's strength. So far they share a civil relationship, however, Camio may have a dislike to him after learning that he murdered Solomon. Though, unlike Sytry, he tried to solve the issue and almost came close to finding out the reason for Solomon's murder. He is also shocked by Dantalion's change.
Dantalion was the first demon to be summoned by Solomon, and although Dantalion had hatred for Solomon in the beginning, he soon becomes close to him. As Solomon's familiar, Dantalion was required to do whatever Solomon wanted. When Solomon ordered Dantalion to kill him, Dantalion was forced to and was deeply saddened to have killed him.
Baphomet introduced himself as Dantalion's butler, but he was a dear friend to Dantalion and a member of his Household or Family. Dantalion valuedhim and trusted him deeply, and was crushed by his death - seemingly going through an entire personality change.
Baphomet trusted and valued for Dantalion in turn, going as far as to say that he should abandon him as his weak state was in turn weakening Dantalion, and giving his life to protect William Twining - another person precious to Dantalion.
Gilgamesh considered Dantalion a friend and Dantalion viewed him as an annoyance. After Baphomet's death Gilgamesh offered to Dantalion to join his Household, and they were portrayed as rather close after that. Dantalion trusts Gilgamesh enough to ask him to protect William.