He's quiet and scary. Every couple of days, Ahzrukhal sends Charon out to run some kind of errand. He always leaves packed with weaponry, and returns with a bag of Caps. He's loyal to Ahzrukhal beyond question and will do whatever his employer orders.
Although little is said about Charon's past, Ahzrukhal will tell the Lone Wanderer that he was raised by a group of people who brainwashed him into obeying whoever has his contract. Charon is one of the most enigmatic characters, especially for a companion. His background doesn't seem to be something that we discuss much.
Ahzrukhal insists that he is not a slave, and implies that Charon did something in the past to deserve his "employment" with Ahzrukhal While the terms of the contract are never shown, if Charon is asked to retrieve the G.E.C.K. in Vault 87, he says that he is only good for combat services and that he is "nobody's errand boy."
At the Ninth Circle, he is the bouncer. He stands in the corner until needed. If dismissed by the Lone Wanderer, he will return there and sit in a nearby chair.
Even though hiring him has no Karma requirements, Charon follows a personal moral path. Stealing in Underworld turns him hostile. If the Lone Wanderer kills a good non-player character in Charon's presence, Charon will become upset and, if subsequently fired, turn hostile.
"Ahzrukhal was an evil bastard. So long as he held my contract, I was honor bound to do as he commanded."
"Physical violence on your part invalidates our contract."
"You are my employer and I will do as you command."
"You purchased my contract from Ahzrukhal? So, I am no longer in his service. That is good to know."
"Talk. To. Ahzrukhal."
What we know:
Charon has been brainwashed as a soldier or a bodyguard.
Charon's conditioning does not involve obeying orders outside combat situations.
Charon still has a personality, he is just very stoic.
What we don't know:
Whether his conditioning took place before or after the war.
What his original nationality is, if he was trained pre-War. (Any regional accent would have disappeared over 100 years)
Why he uses a more archaic form of English ("shall," "once again," no contractions, etc.) in dialogue if he was a 1940s soldier or secret service.
What it's possible to assume from in-game mechanics
He is not able to hold his own contract. This may be an in-game oversight or a mechanic they didn't want to deal with.
He was trained in medium-and close-quarters, as he will always prefer a shotgun over anything with more long-range precision. This may just be a personal preference.
He may have been physically changed during his conditioning: he is much taller than all other human or ghoul characters. While this could be an attempt to make him distinct from other ghoul characters, it still bears consideration.
What are ghouls?
Ghouls are mutated humans or animals affected by the phenomenon of ghoulification. In the case of humans, ghouls are sometimes referred to as necrotic post-humansin the Capital Wasteland but this term does not seem to have been adopted outside of it whatsoever. Despite their Zombie-like appearance, the flesh of ghouls is not actually rotten. Intense and prolonged Radiation has ravaged their skin, much of their flesh, and in some cases many of their ligaments. Despite their infertility and in some cases mental deterioration, they have greatly extended overall lifespans and are immune to (and sometimes even healed by) background radiation and/or nuclear fallout.
Most ghouls were "created" in the Great War of 2077 Ghouls are alive during Fallout 76 (2102- 2103), Fallout (2161), Fallout 2 (2241), Fallout 3 (2277), Fallout New Vegas (2281) and Fallout 4 (2287). All ghouls live considerably longer than normal humans, though they are largely sterile, so much so that the birth of Monica, an unaffected human child of two ghoul parents, was described as miraculous. The unnaturally long lifespan of a ghoul is also due to a mutation within the autonomic nervous system of certain individuals following exposure to specific combinations of ionizing radiation with wavelengths below ten picometers. Radiation that has such a short wavelength, is known as gamma radiation and is normally lethal to healthy humans in even moderate doses. The mutation in response to gamma radiation that produces ghouls disrupts the normal process of decay in the neurotransmitters along the spinal cord.
High levels of radiation are a crucial factor, but radiation poisoning typically results in death, rather than ghoulification. There are two primary ways in which ghouls can develop from a human. Irradiation and gradual transformation result in ghouls who retain their general human shape, but with a high degree of skin deterioration: Exposed muscle, flaking skin, and damaged connective tissue characterized by the absence of nose and ears.Another is surviving a nuclear blast relatively close to ground zero, suffering burns both thermal and radiation-induced, but surviving. The latter transformation is typically faster and substantially more traumatic for the victim and results in a generally much more horrifying appearance, with exposed bone, asymmetric deformations, and extensive unhealed wounds.
Ghouls retain their normal cognition and are generally no different than humans in terms of intellect. In fact, many ghouls benefit from their extreme lifespan and are capable of amassing skill and knowledge far beyond the ability of regular humans. By extension, ghouls retain their sex drive and some humans have a ghoul fetish. Regardless of their capacity for intercourse, ghouls are sterile and cannot procreate. Any children they may have are either adopted or were mutated into ghouls alongside their parents.