Hypnos is the embodiment of sleep. He is one of Nyx' many children, and is a brother to Thanatos, Megaera, Alecto, and Tisiphone.
He watches over the procession of newly-arrived shades coming before Hades, checking their names off his list. He is often seen sleeping on the job.
He greets Zagreus whenever the prince is killed and returns to the House via the Pool of Styx, and often gives unhelpful advice and a quip about whichever enemy or natural force did Zagreus in.
If given Ambrosia, Hypnos will give you the Chthonic Coin Purse.
||'...It is said that Sleep is the cousin of Death. That turns out to be not quite correct, insofar as the two, in fact, are brothers, born to Nyx herself. The former of the two, we call him Hypnos; he whiles away, half-sleeping, watching over the Pool of Styx in the foyer of the House, taking stock of those arriving in the Master's domain, or returning to it. Though Hypnos may be lacking in propriety, he brings a certain cheer, however unconventional, into the grim proceedings of the House. It is a wonder that the Master is so tolerant of this. Some say it is because the Master is oblivious to Hypnos, but it is my choice to think the Master secretly and silently enjoys the bit of levity the fellow purposely or inadvertently provides.'
For additional information on Hypnos that does not pertain to Hades, see Wikipedia's article: Hypnos