A main character of Shadows of the Damned, Johnson was originally a demon, living in the City of the Damned in Hell under the rule of the incredibly evil Fleming. At this time, he had a flesh-and-blood body like everyone else, and was known to frequent demonic brothels, with the Angel's Kiss being his favourite. The exact nature of how Johnson came to leave Hell is unclear; at times it sounds like he was cursed for some sort of failure and banished from Hell, while at others he makes it sound like he personally sought a better existence than the miserable life of a demon and fled Hell deliberately to find freedom, inspired by a legendary heroine known as the Unbreakable Huntress.
Regardless of whether it was a punishment or a price paid for freedom, at the time Johnson left Hell, his body was stripped from him, leaving his entire being a floating, burning skull, decidedly similar to the Lost Souls of Doom, but with a British accent. He does, however, retain a bit of his former power: Johnson has the ability to shapeshift into various fully-functional items, primarily firearms, and due to his nature, these items are visually unique, with a bizarre, demonic style.
Some time after leaving Hell, Johnson met the demon hunter Garcia Hotspur, and somehow the two began a partnership that eventually became a friendship, with the sarcastic skull becoming Hotspur's sidekick, using his shape-shifting abilities to help his partner. The forms he was able to assume at the beginning of the game, and thus the ones he was able to assume on Earth to help Garcia, were of a revolver known as the Boner, a shotgun known as the Monocussioner, a motorcycle, and, the form he assumes most often during Shadows of the Damned, a torch, which really just consists of a handle extending from Johnson's natural form, using his flames to light the way for Garcia. He can also fire a Light Shot from the form of any firearm, which can cleanse a demon of the supernatural Darkness which gives them strength and invulnerability, and freeze a normal demon in his tracks for a short time.
In Shadows of the Damned, when Garcia ventures into Hell to rescue his beloved Paula, who has been kidnapped by Fleming, Johnson reluctantly accompanies him back to the City of the Damned, and uses his knowledge of Hell and demonic society to serve as a sort of guide for Garcia in the Underworld. Also, Johnson can acquire two kinds of gems that can make him more powerful than he ever was on Earth; red gems which can improve the abilities of his current forms, such as increasing the damage dealt by the Boner or the length of time the Light Shot stuns a demon, and blue gems, which greatly enhance his power, allowing him to assume new forms, such as a machine gun called the Teether, and more powerful versions of existing forms, such as the Skullcussioner, a superior shotgun to the Monocussioner. Finally, if Garcia calls a phone sex line from the Angel's Kiss for Johnson, the skull will get so worked up that his revolver form will transform into an incredibly powerful cannon called the Big Boner, though he can only assume this form for a relatively short period of time.
When he is a weapon, Johnson still requires ammunition to fire, just like any other gun, though it isn't clear whether or not the motorcycle requires fuel. In addition, because of the unusual nature of his ammo (revolvers fire demon bones, shotguns fire demon skulls, and machine guns fire demon teeth), it is unknown where Garcia gathered such ammo while on Earth.
Johnson adores strawberries, and apparently abstains from eating meat, especially Human flesh. According to him, he's a 'fruitarian', though he also consumes red gems.