Akuma are created through the mixture of machine, soul, and despair. When a loved one has died and the living still mourn for them, the Millennium Earl appears at their side. He asks for their cooperation to bring back the soul of their loved one and places it in a special mechanical body (魔導式ボディ) that the Earl had created. It's only once this is done that the mourners realize their terrible mistake. Once they bring back their loved ones by yelling out their name in front of the mechanical body, the Earl orders the mechanical body to 'wear' their resurrector's skin by killing them. The soul inside of the Akuma has no freedom whatsoever, and is doomed to follow the Millennium Earl's orders for all eternity unless it is freed by destruction from an exorcist. Cases of self destruction possible by direct command from the Earl or Noah members simply destroys the soul rather than freeing it whatsoever. The Akuma are really soldiers in a war for Innocence against the Exorcists. They hide in the human world by taking over a human's body. It appears that after a while, most akuma upon reaching past level 1 or so cannot remember their original human identities. Only Allen can distinguish between Akuma and humans with the power of his cursed left eye.
Level 0 is the Akuma right after being created. It is only the metal skeleton which is presented similarly to a model kit format when still unoccupied by a soul. The human soul is the dominant personality, but it is still bound to the Earl's control. The ends of the arms varies from Akuma to Akuma. The separate 'bones' are not connected in any way, but rather hover near where they would be connected. After a soul occupies the skeleton, a pentacle along with its host's name below it will be inscribed on the forehead of the skeletal structure. No matter how inhuman an Akuma becomes upon reaching level 2, the level 0 form remains as its skeleton.
Level 1 consists of a floating ball shaped creature with dozens of gun barrels protruding from it. The gun barrels are flexible and can be bent to face any desired angle without affecting its firing capabilities. The bullets made of the blood of the Akuma is deadly to the human flesh, leaving black star marks, cracking and corroding the human's body. Only those with a parasitic Innocence like Allen Walker, Arystar Krory, Suman Dark, etc. can heal the virus, (for Allen, his Innocence just cures him of the akuma poison. Krory can also suck the virus out of other humans that were hit by an akuma blood bullet), from these bullets. The souls inside a Level 1 Akuma have the bodies and clothes of their original forms. However, their faces are rather distorted and there is a chain around their necks expressing intense pain and sadness of the trapped soul. Level 1 akuma exhibit somewhat a transition from deteriorating human personality to an evil personality for level 2 later on as observed in their increasing obsession to simply kill and may, hence, show zombie like behaviour in their human form. A Level 1 Akuma will advance to a Level 2 once it has fulfilled a substantial quota of kills of humans and exorcists.
Level 1.5 is not an actual level per se, it is more of a mid-way point for a Level 1 Akuma. The only difference is that the Akuma becomes rounder and more spherical, with the ends of the barrels on the surface. Their facial expressions appears to show more instances of emotions closing in to a personality as of a level 2 (In the anime, a level 1.5 akuma grins like a maniac when it successfully anticipates and counters Allen Walkers attack from behind a building.) Their bullets can also penetrate barriers that normal Level 1 Akuma bullets can't.
Level 2 produces a dramatic change. They develop a unique ability and a sense of self. Level 2 Akuma are capable of intelligent thought and are largely independent, though they still maintain unquestionable loyalty to the Millennium Earl and the Clan of Noah. The souls of Level 2 Akuma seem to become more deteriorated. The souls inside of these Akuma can't seem to be identified at all and are in a fetal position, wrapped from head to toe in bandages. Level 2 Akuma come in all shapes and sizes (some have even appeared simply as a gas, fire, a demonic court jester, or even a playing card) and they have no common appearance. They also have different powers, such as copying a person's appearance (mirror image), emitting "ice fire" which hurts worse than normal fire, creating blades of air, emitting destructive sound waves, and creating bubbles that absorb water from everything they touch. Some Level 2 Akuma even carry modern day weaponry such as homing missiles and various types of firearms that resemble machine guns and so on, as observed in the manga. Level 2 Akuma are able to fight on the same level of a normal Exorcist, unlike their previous Level 1 form which was easy for Exorcists to destroy. After murdering enough people, it can evolve to Level 3.
Level 3 brings perhaps the most terrifying changes of any previous stage. Regardless of their appearance in Level 2, all level 3 Akuma become bipedal, roughly 8 to 9 feet tall, with sharp jagged teeth, and wear what looks like medieval armor that covers most of their bodies (other than a few exceptions). In addition to this, they receive a massive power boost. A Level 3 Akuma is easily capable of standing up to multiple exorcists in combat and usually has an extremely powerful ability in addition to its greatly improved strength and speed. Their abilities seem to revolve around the use of their Dark Matter, the polar opposite of Innocence. General Level Exorcists, or Exorcists with powerful Innocence are able to destroy Level 3 Akuma easily though. The first Level 3 Akuma encountered in the series was only destroyed after a fight that almost cost Lenalee her life and forced her Innocence to take drastic measures, for a shocking revelation. According to Cross's modified Akuma, there are plenty of Akuma at Level 3 or above in Japan. The souls embedded in the level 3 Akuma are near gone and is a floating dark mass with eyes. The souls want to be released from being an Akuma by saying phrases like "Please love me," or "Please destroy me,". There are different types of Level 3 Akuma, one of which is the Stable, Melee-fighter type.
Level 3.5 like Level 1.5 also serves more like a half-way step for the evolution of a Level 3 Akuma, except that the Akuma's form and shape no longer bears any similarity with its former Level 3 form. Instead, as the Akuma starts to approach level 4, it tears off and sheds all of its metal skin and mechanical components and begins to develop or sprout organic flesh that quickly envelops the skeleton of the former Level 3 Akuma. The mass of flesh grows in size and begins to take human form, eventually transforming into a roughly 15 feet tall, horned, nude, and human-shaped female. This form has no feet, as it stands erect like a statue on top of a pile of other Akuma and human corpses, and its belly is largely swollen giving off the impression that it is pregnant, with a number '4' in bold clearly painted on its stomach. Unlike previous Akuma forms, the Level 3.5 form remains completely motionless in a praying position, thus, it acts more like a chrysalis and shell for the evolving Akuma inside, which eventually it bursts out from the shell's womb in a contrasting act to human birth.
Level 4 The rarest Akuma's evolution yet (Bookman stated that in over 100 years, the order and Bookmen had never confirmed a level 4). The soul trapped inside a Level 4, according to Allen, doesn't even look like a soul anymore and is surrounded by a veil of darkness. Its form is so horrifying that it is not even shown (Allen vomited after witnessing the horrifying change, unable to look at it after his initial glance). Unlike the tall, imposing figure of a Level 3 Akuma, the first Level 4 Akuma introduced is about the size of a small child (between 4-5 feet tall), and possesses a more childlike appearance. The body is pale, and the appearance of the face is similar to that of a combined Akuma. The body has an 'organic' appearance, with a series of large and small stripes ringing its arms, legs, and neck. A large, inverted pentacle appears on its slightly bulged stomach, with a smaller pentacle on its forehead. The joints on a Level 4 are visible, similar to the joints on a wooden puppet. A Level 4 also possesses a pair of small fairy-like wings and a pair of flattened halos over its head (one small, one large), giving it an angelic appearance. The Akuma's innocent appearance is just a facade, however, as it refers to itself as a 'massacre weapon'. A small flick of its fingers is strong enough to send a General-Level Exorcist (Allen) flying what seems to be several meters, with the consecutive landing enough to knock them unconscious and put a rather large crater into the impacted wall. It can also make a scream that is able to affect the generals and other exorcists and disrupt their synchronization rates. It has shown little regard for members of its own faction, as it killed many of the newly made Skulls, all because it was "a little excited". It also seems to have the ability of spreading the akuma blood virus by contact. Lenalee and Allen were the first two exorcists to defeat a level 4 akuma and, even with their combined effort, it required the assistance of General Cross Marian to destroy the akuma completely (though General Cross seemed to be able to overpower the level 4 Akuma with little effort shown).
It has been shown that the Millennium Earl can combine a large quantity of Akuma (perhaps millions) into several dozen larger Akuma. Several were defeated, but were not quite stronger than the sum of their parts. However, considering that the eye of one of the slain of this version of akuma attacked Johnny Gill and later evolved into a level 4 akuma, it might be possible that these akuma have more abilities than originally shown.