Hypnos

Hypnos is the personification of sleep in the ancient mythology and a son of Nyx and Erebus. The brother of Thanatos, he is the minor Olympian god of sleep. He has a throne on Olympus made out of clouds. He has three sons: Phobetor, Phantasos and Morpheus, who are all sleep and dream specialists. His Roman counterpart is Somnus. He has at least three sons at Camp Half-Blood: Clovis and two others. Hypnos is probably where the words 'hypnotism' and 'hypnosis' are derived from.

The Last Olympian
In The Last Olympian, Hypnos has a cabin at Camp Half-Blood probably after he has his own throne on Olympus when the Battle of Manhattan has gone. He gets his own throne after Percy makes all the Olympian gods and goddesses vow to an oath to the River Styx that they should honor and claim their demigod children when their children reach the age of thirteen just so that no other unclaimed demigod gets frustrated with the gods and decide to cause another Great War such as the Second Titan War that ended with Luke's sacrificial death so that Kronos, Titan Lord of Time and possessor of his body, could not end up destroying Mount Olympus and that Olympus could survive.

The Lost Hero
Annabeth takes Jason, son of Zeus's Roman counterpart, Jupiter, to Hypnos' cabin at Camp Half-Blood when she realizes that Jason's memories are lost from his mind and that Clovis, Hypnos' demigod son, has the solution to why his memories are lost. There, Jason feels that he is starting to fall asleep and Annabeth tells him that even though Hypnos is the minor Olympian God of Sleep, the inside of his cabin makes everyone fall asleep. They find Clovis asleep and Annabeth tries to awaken him, but he didn't hear her at all. So Annabeth says his name louder, making Clovis jump at the loudness of her voice and he gets up, half-awake. Once he hears Annabeth explain Jason's problem of lost memories, Clovis instantly awakens further and hears the story again.

Personality
Hypnos is said to do nothing much but sleep, but Somnus is said to have liked killing people if they slept during their jobs.