London is a master vampire of Belle Morte's line who moved to St. Louis before the events of Danse Macabre. He is often called "The Dark Knight". He is scary, proud, cynical and broody, and sometimes intentionally cruel, and Anita doesn't like him very well.
His powers allow him to feed the ardeur every two hours, day after day, without any ill effects to his own health. On the contrary, he gains power from being food. It could make him a particularly effective pomme de sang if he only got better along with Anita. As a downside his powers also make him immediately addicted to the ardeur, whereas for most people addiction builds over time.
London has short, dark curls and black eyes, and prefers black clothes. He likes to brood in shadowy corners, and does not smile or laugh unless he's high from ardeur. Anita thinks he looks like a cross between an executive Goth and a movie hit man.
Before the series
It's unclear who was London's original master, but he was turned a few centuries before Jean-Claude. At some point London was traded to Belle Morte's court for several years, until she exiled him. At some later point she tried to recall him, but he got his new master to intercede. It's unclear whether this new master was already Dracula, the Master of the City of London, or whether London the vampire moved to the city of London at some later point in history. He later claims to have been disappointed too many times in too many different masters as an excuse for his cynicism.
London is not personally mentioned in this novel, but we learn that Dracula has recently gone off the rails and been executed. Belle Morte has consequently attempted to demand the return of every vampire of her bloodline who was in Dracula's court, but Jean-Claude has offered many of them an alternative and they have flocked to St. Louis instead. It's unclear whether London is part of this initial group or arrives later.
We learn in this novel that London begged for a sanctuary against Belle Morte after the mess with Dracula. London didn't want to go back to her because he's a recovering addict to ardeur and knows from experience how she would use his addiction against him. He is so intent on avoiding falling back to his addiction that he even arranged an escape clause to his deal with Jean-Claude, in case being near other versions of ardeur would turn out to be too much. In such case Jean-Claude would find London another master to hide behind, although he has too much potential power to make that an easy task.
London is distrustful of Anita, especially after she unintentionally addicts Requiem, and even more unnerved after Anita uses her necromancy when she's trying to free Requiem's mind. London leaves her presence at the first opportunity, but doesn't go far enough, as at a later point in the novel Anita is in desperate need to feed the ardeur and it finds and brings London to her. He is powerless to resist her call, even if he's terrified, and he feeds Anita's ardeur unlike anyone ever before. Afterwards he tries and fails to hide how high he is from the experience, and confesses to feeling better than in centuries.
Even later London's addiction is confirmed rekindled. He brushes off Meng Die in the process, angering her further, as she blames Anita for stealing all the men. We finally learn of London's special powers regarding ardeur, and that Jean-Claude had kept them as a secret in respect to London's wishes, even if he could be just what Anita needs. London seems now happy about being addicted again, more at home in his own skin than before, but it's clear to others that it's the addiction speaking.
At the end of the novel Anita still isn't sure what to do with London, as he doesn't seem very well suited into their domestic group, even if his powers put him as the front-runner for her new pomme de sang.
In this novel London's status still hasn't clarified, but he's still in Anita's feeding roster, and still glowing from addiction. He is present but not in very big role in several scenes, and mentioned on and off. Anita tries to claim he hasn't gained power from sleeping with her, but this is shot down with the observation that London was already a master vampire and it's difficult to tell.
We don't see London in this novel, but he's mentioned twice as regular food for Anita's ardeur.
Again we don't see London in person, but he is among the group that follows Anita to Las Vegas, and is mentioned a couple of times as her not-quite pomme de sang. Anita calls him a good lover but bad boyfriend.
In this novel we learn that London has been sent to visiting other vampire territories as a potential second-in-command, and is currently on his fourth such trip. He has gained so much power from being Anita's food that he is quickly approaching Jean-Claude's level, and Anita has had to step back. London is already powerful enough to hold his own territory, but not good enough in modern politics to make it advisable. We don't see him in person this time either.