Maria is Camio's lover. Despite being apart for at least 20 years, neither lost feelings for the other. Because Maria is dying, Camio thought to make her into a Nephilim.
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 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 (recently, he has taken an active role to be elected, and has expressed a desire to be Emperor).
Uriel and Camio, despite being on opposing sides and personal conflict in the past, have a very civil relationship. They have been seen conversing with each other even after they learned of each others true identity.
Camio in spite of very few interaction with Dantalion shows he has respect for Dantalion in spite of his Nephilim status. Dantalion in turn acknowledge Camio's strength. So far they share 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.
As of the most recent Pillar 77, Camio is not the reason that Baphomet has died, not entirely. It may have been part of his plan, but we learned the masterminds behind it all are Lucifer and Samael. They are attempting to throw the two into war, to determine who should sit on the throne via power, not an election or support. We see that Dantalion stops Camio from destroying the world after losing Maria, and see them join forces. It can be implied that even if they are not friends, they no longer dislike each other as they had before.
John Dee and Camio met about 400-500 years prior to the manga. John Dee works as Camio's butler, but appears to mainly look after their house while Camio is away. John Dee shares perhaps the closest relationship to Camio, even more so than Maria. John Dee also manipulates others into believing what he wants them to, as seen in more recent chapters. Camio met John Dee when John Dee was a child, and later after he became a teenager. John Dee 'forced' Camio into being his friend by threatening his life. After that, Camio attended the same school as John Dee, until he disappeared from John Dee's life. Once John Dee's life was near the end, Camio came back and made a contract with him making him his Family, much the same way Baphomet is bonded to with Dantalion. John Dee can be assumed to be one of the few who knows most of Camio, as he reveals his knowledge about Camio's past, something Camio didn't even tell Maria.
Lucifer hasn been shown to care much about Camio, and is well aware ofhis wish to kill him.
Cassandra, Queen of Troy, was Camio's mother. He loved her very much. Her situation and fate after meeting Lucifer is the reason Camio hates his father.
Not much is known of what Camio and Sytry think of each other. They do not interact directly with each other much at all. It is known that they do know of each other, and Camio at most would be dismissive of Sytry. However, once they tried getting Dantalion to confess that he had murdered Solomon with his own hands, they appeared to be together. At most, we can conclude that Sytry probably admires Camio a bit, like he does Dantalion, even though they are each others rivals.