Lucien was originally the first Raven companion to Dream of the Endless.

Lucien was altered or evolved into a humanoid form, taller even than Dream, and with pointed ears. It is entirely possible he might have been a mortal once, just as his successor as Dream's Raven, Matthew, was. Lucien serves on Dream's palace staff as the chief librarian, overseeing a collection of every book that has ever been imagined--even if that book was never published or even written.

When Dream was captured by the magician Roderick Burgess and his Order of Ancient Mysteries, in the first decades of the 20th century, Lucien was the only member of Dream's staff who did not abandon his post, and tried to maintain the integrity of the Dreaming as much as was possible in its maker's absence.
When Dream returned, it was Lucien who alone greeted him at the decayed and crumbling palace with the comment, "Breaks your heart, my Lord, doesn't it?"  He caught Dream up on the unraveling of his realm, what had vanished - including the entire library;  and who - including the Fashion Thing, who had been missing for a year, and Brute and Glob, who had vanished two decades past.  He empathized with Dream over the diminishing of his realm, "It hurts me too, Lord"

As a result of Lucien's loyalty and tenacity, Dream increasingly relied on his counsel, and Lucien's responsibilities expanded accordingly. He is Morpheus's most trusted servant. When Morpheus leaves the Dreaming, he sometimes gives Lucien authority to oversee matters while he is gone.