Hell's Angel a.k.a The Headless Horseman
"I am the Hitman for God. I am the thing that the monsters under the bed fear."
-The Headless Horseman, Telepathically
Karl Mueller is the world's most prolific serial killer with the possible exception of Blood Wing. If you count all the various people that he's killed since the Revolutionary War, then his body count may be somewhere close to 5,000 people. The vast majority of these occurred after his death and were murders, rapists, or worse. Thus, the Headless Horseman has a decidedly mixed reputation amongst the people of the world. For the most part, people consider the Hell Spawned Damned Avenger to be an enemy of the world.
In life, Karl was a horrifically bad man. Even he doesn't dispute this. His crimes as a Hessian mercenary working for the British are some of the worst atrocities that occurred during the Revolutionary War. Karl has no excuse for his actions. He enjoyed bloodshed and murder like some people enjoyed fine wine. When his head was taken off by cannon fire, he was sentenced to a well deserved punishment in the fires of Hell.
Karl would have stayed there if not for Merlin the Magician. Why Merlin chose Karl of all people to be his Demon Hunting Ghost is anyone's guess. However, Karl served Merlin through countless wars and mystical conflicts. Each time, Karl hoped Merlin would reward him with freedom or a second shot at life. Merlin, honestly, didn't care to do either. He considered Karl to be his slave and a man who deserved no mercy. Karl would never forget this dismissal and it still burns him to this day.
Since breaking free in the early 1900s, Karl has been a changed man. Obsessed with not being returned to Hell, he also feels genuine remorse for the majority of his crimes. This has not lessened the fact that Karl is an uncontrollable berserker. Once battle begins, Karl isn't sure he can stop and often ends up destroying everything around him in an orgy of Hellfire. Oddly, since his freedom, Karl has never taken an innocent life save under magical control. The act of doing so will cause Karl to hunt an individual literally to the ends of the cosmos.
Ehrich Weiss, against his better judgment, helped Karl come to terms with his newly Un-Damned existence. During World War 2, Karl became the enemy of his own people and realized that he had the ability to see the souls of human beings and know instinctively who was beyond redemption. He briefly fell under the control of Count Reich II but was a man who wrecked a terrible vengeance on the Nazis afterward. He also performed the first genuinely selfless deed in his life by releasing a concentration camp's victims.
Peculiarly, Karl then developed the ability to assume a human form during the day with all of the benefits of being alive. Karl has since taken this as a sign that God, or whatever rules the cosmos, wants him to use his ability to save lives rather than take them. Unfortunately, Karl is just not very good at saving lives rather than taking them. As a compromise, he focuses the majority of his unending hatred and rage against monsters rather than people. Demons, Vampires, and Satanists fear Hell's Angel as the implaccable wrath of God.
Karl is an uneasy fit in the Tomorrow Society. He's a major reason that they're viewed as a terrorist group. However, the Red Queen can't dispute the fact that Karl has saved their lives on numerous occasions. He's also done what the Red Queen can't in occasionally taken the lives of those would destroy all of Metakind. Really, the Hell's Angel is all over the Halt Evil Doer! setting. Because Karl becomes a monster with every sunset, he has no choice but to seek battle whenever he can.
Nomadic: Karl becomes a headless undead spectral being every sunset, so he's not exactly a man who can hang out at bars. Instead, he mostly spends his time traveling across the United States on his bike and searching for evil to destroy. Karl doesn't sleep and this gives him extra time to deal with his enemies.
The Kalderash Circus: A group of Romani, Karl is politically incorrect by just calling them "Gypsies", who run a carnival that travels across the United States. Contrary to stereotype, none of them have innate supernatural powers except for their curious association with Hell's Angel. Instead, Karl made a promise to a little girl of their clan during the time he was alive and has honored the promise across the centuries. This was a promise to protect their people from retribution and he's done so against everything from Gangsters to Nazis. In part, thanks to the Horseman, the clan has taken in refugees from the supernatural world like lost psychics and people who have escaped pacts with the Inferno.
John Milton Miller: Karl's son. Karl was unaware that he could even have children but it happened anyway. John is the product of a liason during the day in the 1980s. John's mother was a young street hooker that Karl rescued from her pimp. John has none of Karl's powers but for a supernatural sense that prevents him from living a normal life. John doesn't have much of an affection for his father and spends most of his time investigating occult crimes. Karl has a lot of regrets regarding John but doesn't really know how to make it up for them. John is actually starting to look older than his father and its rather annoying him.
If Karl were ever to be destroyed, all of his powers would transfer to John. A fact that has occurred a few times when Karl has come close to being destroyed forever. It's something both are anxious to avoid.
Black Wing II: Karl hates Alex. Alex hates Karl. They're both civil enough around each other when Serena is present but they have a barely understated loathing of the other. Part of it is over Serena's affections and the rest is simply their personalities. Alex considers Karl a dangerous loose cannon who should be in prison. Karl has a lot of lingering class resentment against American aristocrats, modern or old.
The only reason Black Wing II doesn't try to take in Karl is because he's aware someone has to thin the herd of vampires and other monsters in the world. That and Serena would never forgive him.
Ehrich Weiss: Karl is grateful to Ehrich and incredibly differential. Aside from Ehrich's occasional annoyance at Karl's political incorrectness of referring to him as "A credit to the Jews" and other things that Mueller doesn't realize are no longer considered compliments (if they ever were), they actually get along rather well. Ehrich considers Karl just barely above a psychopath but that was the way he was raised and the product of his environment. Ehrich believes, hopes even, that Karl might somehow achieve the redemption he seeks.
In the meantime, whenever he shows up, Harry Houdini tries to reign in Karl's actions and direct them to more productive ends.
Red Queen: Karl loves Serena. Serena tends to view him in a manner that varies from being flattered to considering him stalking her. Still, she cannot deny the demonic part of her nature is attracted to his human side. Karl follows her orders in no small part due to his attraction to her and eventually hopes to wear down her resistance.
In the meantime, he has no problem skirt chasing. A fact that exasperates Serena who comes from a decidedly more conservative morality.
Ultimatum: Ultimatum dislikes Karl. While he's viewed as a criminal unfairly, Karl really IS a mass murderer. On the other hand, Karl has often shown there is a very real hero underneath his frightening exterior. Karl thinks Ultimatum is a "nice kid who'll make girls happy when he hits puberty." That, of course, has lead to Ultimatum beating him down repeatedly.
Valkyrie: Valkyrie is Karl's sidekick. This is something that has been shown in countless Headless Horseman comics and is pretty similiar to the Wolverine/Kitty Pryde relationship. Karl considers her an innocent and she finds him to be a peculiar totem to hang onto. Madame Tomorrow is a bit suspicious at the fact that Karl will live forever and women Jun's age aren't necessarily beyond the age of a relationship in the Horseman's time. Karl, himself, would find this jealousy from her to be amusing.
Dracula: It would be wrong to say that Dracula fears Hell's Angel. Instead, it would be better to say that Dracula does not underestimate the man. Karl has defeated many of Dracula's spawn over the centuries, including some of the most powerful in the world. He eventually hopes to kill the most powerful of all vampires. Dracula knows, should Hell's Angel do the deed, then he will finally be banished to Hell and will never be able to return to life.
Lamia: Also known as Lilith, the Mother of all Demons. Karl hates the Queen of the Succubi with a burning passion and has destroyed countless Lilin over the centuries. She hates him with an equal burning passion. Lamia's appearances are things that push the Comic Book Code Authority's tolerance for sensuality.
Lamia, contrary to Hebrew Mythology, is Orkus' sister rather than a genuine demon.
Mister Hoppy: Mister Hoppy's relationship with Hell's Angel is complex. Honestly, Mister Hoppy wants to lure him into Hell's service. Karl has been offered his humanity back and several other very lucrative deals over the time. A few of the selfish ones, Karl has taken and been eventually forced to reverse because of his developing conscience. Still, Mister Hoppy often shows up at Karl's various hideouts with new deals to torment him with.
A typical arrangement will usually have Mister Hoppy use Karl to eliminate a wayward demon that has escaped his control. In exchange, Mister Hoppy will cure the cancer of someone Karl likes or give Karl a chance to be with a lover at night for a couple of days. Their relationship is oddly similiar to the one between Ezekiel Stone and Julian Glover's Devil on the TV show Brimstone.
Merlin: Hell's Angel will kill Merlin if he can. Flat out. The hatred for the humiliations and acts that Merlin forced him into means that he's tried (and failed) in every one of their encounters. Only the destruction of the world being on the line will ever remotely compel the Horseman to put aside his loathing for even a short while.
Orkus: Karl's rather limited supernatural education assumes that Orkus is in fact the Devil. Orkus is powerful enough to destroy Karl outright. However, instead, the Warlord of Abaddon prefers to dispatch his myriad servants against him as a way to test them. Hell's Angel has managed to destroy some of Orkus' avatars with the help of the Tomorrow Society but he is way out of his league against the real thing.