A minor goddess in Greek mythology, Pasithea was one of the Graces, associated with relaxation and hallucinations. And being the wife of Hypnos, the god of sleep, would also make her the sister-in-law of Thanatos, the god of death. As well as related somehow (depending on your sources) to the various personifications of dreams and nightmares, including Morpheus after whom morphine is named.

All pretty appropriate for the drug, I reckon.