Melchior (pronounced something like Mill-Key-Or, not Mel-Core like Anita says it) is an ancient vampire and a master jeweler, who is as passionate about his craft as his human servant and assistant Irene. He is especially skilled as a metal worker.
We have yet to see Melchior in flesh, but his body language while possessing Irene hints that he is taller and beefier (not surprising, given that Irene is tiny), and has a beard.
Melchior has been an artist for thousands of years, and has an attitude to match it. He met his human servant in ancient Mesopotamia, and they have created marvels of jewelry together through centuries. He is not only her vampire master, but also her master jeweler.
History within the series
In Dead Ice Melchior (together with Irene) has been contracted to design rings for Jean-Claude and Anita. He lets Irene show them the ring designs on daytime, even if he can't be dead to the world at least past the beginning of the meeting, as he takes over her body after Jean-Claude lets his powers of seduction affect her.
It's unclear whether Melchior's spirit possession skills are limited to his human servant or can be used to take over other bodies as well, but whatever the case he isn't as good at it as the Traveler. Melchior treats Irene's body as if it's his own male shape, and doesn't notice when her feet get twisted into a position that lacks balance.
Melchior doesn't have much respect for Anita. He thinks her attempts to participate in the discussion mean that she's getting ideas above her station, probably born from Jean-Claude's plans to marry her, and disapproves of her language ("she curses like a stevedore"). Melchior also dismisses Irene's desire to be seduced as irrelevant next to their shared art, and says that she's free to take a lover as long as it doesn't interfere with her work. It's implied that any interfering lover would be killed regardless of Irene's opinion.
Anita declares she doesn't want to wear a ring made by someone who sees her as less than a person, and Jean-Claude back her up, telling Melchior to pack their things. This shocks Melchior, who protests that the designs are almost finished, but Jean-Claude is certain they can start anew with someone else who can create rings of lasting beauty. When he mentions the possibility of diadems and crowns (the first crown to vampirekind in centuries, at that), Melchior seems truly pained, and even more so at the suggestion of going to someone called Carlo, who isn't as skilled with metal but is said to have a better eye for jewels.
While this discussion is ongoing Anita gets to move right next to Irene without Melchior paying Anita any attention. Anita foot-sweeps Irene's legs from under her and catches her before she hits the ground, startling Melchior, although he's still attempting to keep talking to Jean-Claude alone. Anita points out how lacking the connection between Melchior and Irene must be for him to be unable to feel her feet, and threatens to break the connection between the master and the servant for treating her as a slave. Even then Melchior doesn't take Anita seriously, until she shows her own vampire eyes instead of Jean-Claude's and reminds Melchior who killed the Mother of All Darkness, the only person before Anita to be capable of causing such separation without death.
Melchior tries to hide behind vampire laws, pointing out that it's one of their greatest taboos to interfere with another master's human servant, but Anita and Jean-Claude remind him that he is subject to the human laws as well, and that slavery has been illegal there since 1865. Melchior is starting to panic when Jean-Claude asks him whether he loves Irene, and at first Melchior only admits to loving her art and creations, but when Jean-Claude and Anita both push their wills against him Melchior becomes downright poetic concerning Irene. When Melchior's head starts to clear he accuses Jean-Claude of bewitching him into loving Irene, but they convince Melchior that the love was there all along, he'd just been ignoring it -- although when Anita and Jean-Claude discuss the matter later between just the two of them they admit that they aren't absolutely certain it really was so, even if Jean-Claude thinks it probably was.
In the end Melchior says he'll tell Irene of his feelings, and insists on starting anew on the ring designs, because he didn't understand love before and the current designs are too cold. He calls Anita his queen for the first time (whereas Jean-Claude has been his king from the beginning of their visit), and withdraws from Irene. She doesn't remember a thing about the whole incident, but is perfectly content with having been possessed. She is confused when told that Melchior has changed his mind about the designs, but trusts his vision and hurries to discuss it with him.
The book doesn't mention what becomes of the relationship between the master and the servant, but presumably it does happen. At the end of the book the ring designs are still a work in progress, although the crown designs are coming along more quickly.