Immortal Beings are a species which generally reside in the Afterlife and fulfill a specific purpose from which they cannot deviate. Immortal Beings tend to be rigid in their manner of thinking (with the exception of Michael), and all of them have distinct personalities based on the realm from which they originate. They have eternal lifespans and presumably were all created at the beginning of time (although this is disputable, as Michael has referred to some Bad Placers in his neighborhood as "millennials", stating that they've only been torturing people for a thousand years, and Michael has stated that he'd been an apprentice for 200 years before being assigned his first solo project, a fact which was confirmed in flashbacks). Immortal Beings always adhere to a function which helps the bureaucracy of the Afterlife function, although it is unclear whether they exist in some form before their function or whether they must train for the position.
Role in the Universe
Immortal Beings appear to have created the Universe, as the Janet Manual refers to the "makers of light, darkness, and everything." They then set a point system in place to judge humans and reward or punish them. (It is unclear if Gen was one of these Immortal Beings, as she does state that hydrogen was the only thing in existence when she was born.)
Role in the Afterlife
The defining feature of Immortal Beings seems to be their adherence (again, with the exception of Michael) to their specific role in the Afterlife's bureaucracy, and may take interest in the humans' plight but do not seek to change it. (Gen allows an experiment mostly out of boredom, and Jeff allows Michael to escape because he's grateful for the frog mug). While they seem to have distinct personalities, they really seem to have the Afterlife's system in their genetic code.
Of course, Shawn sent many Bad Placers down to Earth to collect the humans, thereby breaking what was presumably a law created by Gen (that Immortal Beings may only venture to Earth through her door), but even this was to reclaim the humans and ensure that the Afterlife's system remained ongoing.
Appearance
All Immortal Beings in the show thus far (with the exception of Toddrick Hemple and Lance) have been seen only in human form, though according to Todd this is because of their human suits, which are really itchy. However, it seems as though the Accountants, at least, are fully human in appearance (by necessity, because of their jobs) because there is no reason why they would be wearing human suits (although the same can be said of Shawn and his Bad Place associates in Chapter 13, Chapter 24, and Chapter 25, as they are not in the presence of any humans and yet still have human form, although this is probably because of what is explained by Michael in Chapter 17, that everyone in the Bad Place Bureau of Human Affairs is randomly assigned a human body so they can "get the feel of how best to torture" them). However, the Accountants have no need for this, nor does Gen, so it is assumed they are either humanoid by choice (in the case of Gen) or necessity (in the case of the Accountants). Good Place Architects probably take human form for the comfort of the residents, as in the case of Beadie, as in Mindy's orientation video she is in human form. It has yet to be confirmed that all Immortal Beings have a non-human form, so it is safe to assume that Accountants may only have human forms. It is also safe to assume that (at least in The Bad Place) Immortal Beings spend the majority of their time in their human forms, as Janets take humanoid form and Bad Janet exists only to serve the Immortal Beings in The Bad Place.
Existence
Immortal Beings appear to have no purpose or existence outside of the process of judgement and reward/punishment of humans. Michael states in Chapter 18: Existential Crisis, after being asked about his hobbies after work, that "there is no after-work"–all of an Immortal Being's time is spent fulfilling their specific purpose. It is unclear whether or how Immortal Beings generate, or whether they were all formed at the beginning of time. (Michael's claimed his birth year is 0000, and that he's been around for all of time, so it's assumed that 0000 is the year on a cosmic sense, or perhaps in the sense of the Jeremy Bearimy Timeline measurements). Because Michael had been an apprentice for 200 years, and "millennials" have only been torturing for 1,000 years, it is possible that Immortal Beings fulfill other purposes before being tortured. This is likely the case, as Michael has stated that there is a Bad Place Bureau of Human Affairs, which implies that a branch of The Bad Place deals with nonhuman affairs. Perhaps being a torturer is a promotion one achieves (so the Bad Place top of the hierarchy would be torturer, then above that, apprentice, then architect, then boss (such as Shawn), then a member of the High Council. Some Immortal Beings, such as Trevor and Beadie, also appear to be representatives (in addition to actively dealing with the humans in their neighborhoods, as Trevor was shown to do in Neighborhood 12358W) for the Bad and Good Places, respectively.
They appear to require no sustenance, although they have been shown to drink antimatter regularly. They also appear to be able to eat, as they are able to eat in Michael's fake neighborhood as part of the ruse. Eleanor speculates that Michael eats babies, although this is never confirmed–however, it does seem likely that some forms of torture does include being eaten. So Immortal Beings can consume, they just don't necessarily need to consume.
Commitment to Point System
Immortal Beings appear to be so apathetic to the plight of the humans and committed to the Afterlife point system because they have never been to Earth–as shown once Gen visits Earth and immediately sympathizes with Michael. Having never left the Afterlife, they blame humans for the choices they make while not understanding the plight of humans.
Hierarchy
The immortal hierarchy
Judge Gen is in charge of managing all conflicts between the Good and Bad Places, as well as significant matters pertaining to Earth or the Afterlife as a whole, and presumably the Neutral Zone. Issues that are specific to the Good Place, Bad Place, or Neutral Zone are overseen by the Good Place Committee, Shawn, and Gen or Neil (respectively), with assistance from their respective Janets.
In the Good Place, Paula is the moderator of a committee which makes decisions as a group. Beadie also works for the Good Place but is not part of the committee; she seems to be something resembling a human relations representative.
A "High Council" is said to oversee the Bad Place, of which Shawn is a member. Shawn appears to make most of the decisions about the Bad Place's affairs, however, and the High Council is yet to make an appearance.
Neil is the head of the Accounting Department, but Judge Gen manages all other Neutral Zone business, such as Janet production and the door to Earth.
Trivia
- There has only been one case of an immortal being becoming a human. This case was Michael, who had been given permission from Judge Gen.