Homunculi

Homunculi are artificial life forms (often, but not always, artificial humans) created through alchemy. Their abilities and traits vary depending on how they were created.

Theory
In the Baccano! series, as in the real world, the idea of a homunculus was popularized by the alchemist Paracelsus. He is said to have created a homunculus which possessed all knowledge from the moment of its creation; however, it was too fragile to live outside its flask. In the real world, it is said that Paracelsus's homunculi had to be kept alive by providing it with blood.

Colony Homunculi
By culturing the cells of a potential homunculus made with his own immortal cells inside a sample of the incomplete liquor of immortality, Szilard Quates was able to create homunculi which are complete immortals but which possess no knowledge at birth. Homunculi created in this way are recognized as "another self" or as an "independent mobile colony" of the Immortal whose cells went into their creation (or of the Immortal who ate their creator). The Immortal can pass knowledge to the homunculus using their left hand, and also has the ability to end the homunculus' life by cutting the connection between their own Immortal cells and the cells unique to the homunculus.

Ennis has described this process thusly: if Szilard is the "company's main store," then Ennis is a branch store. Szilard can "shut her down" at any time.

The biological sex and appearance of the homunculus seems to match the sex and appearance of their genetic donor, but can be changed if another individual's genetic material is donated. In creating Ennis, Szilard used both his own cells and the cells of a mortal woman he kidnapped to catalyze her creation. Inside a cultivation tank, Ennis grew to be the same age as her 'mother' because the culture liquid was the incomplete product, and then stopped growing because she had Szilard's immortal cells. Ennis likely looks identical to her 'mother', just as Melvi Dormentaire looks exactly like Avaro Sr. did in his youth.
 * Homunculi of this type include two male homunculi created by Szilard prior to Ennis's creation; Ennis; and Melvi Dormentaire, who was created by House Dormentaire using Lord Avaro's genetic material.

"Water" Consciousness Homunculi
"Water" consciousness homunculi are a type of hive-mind consciousness derived from Szilard's research into immortality. They are so called because their 'will' resides in a clear liquid that resembles water; however, the "water" itself is not conscious, lacking neurons or anything that could be used to perceive the outside world. Instead, when the "water" is ingested, a battle of wills ensues between the ingestor and the homunculi. If the homunculi wins, they take over the body of the ingestor and inherit all of their knowledge; they are then able to freely control that body. Theoretically, if an ingestor were to win, the ingestor would inherit the knowledge of the homunculus but not take over the rest of the hive mind. This has yet to happen in canon. It is, however, possible for a "water" consciousness homunculus and an ingestor to coexist in the same body under certain circumstances.

These homunculi were originally developed by Szilard Quates and researchers working under him via his research into the way immortality works. They also focused on creating new bodies to serve as their vessels. Additionally, their research was stolen (the implication is that it was stolen by Fermet) and brought to Huey Laforet, who advanced much more quickly by allowing the water homunculi to take over living humans instead of creating blank vessels for them.
 * Homunculi of this type include Fil and Dez/Feldt Nibiru, who were created by Bilt Quates and other researchers, and Sham and Hilton, who were created by Huey Laforet.

The Incomplete Homunculi of the Lamia
The Lamia are homunculi created by Huey Laforet and Salomé Carpenter in Rhythm, Huey's research laboratory. The method of their creation is unknown. They are ageless but not immortal, meaning that they can die from everything except old age. In other words, they can live forever so long as their bodies do not suffer fatal harm. Many of them are eccentric in personality and in physical appearance, but it is unclear whether this is a direct function of how they are made or a result of sadistic experiments conducted on them by Huey. Many of the homunculi deliberately act eccentrically so as to distance themselves from humans.
 * Homunculi of this type include Adele, Christopher Shouldered, Hong Chi-Mei, Rail, Frank, Sickle, The Poet, and a number of unnamed Lamia introduced in 1935-B, including: the tattooed contortionist; the shirtless man; the poisoner; a man wearing a tailcoat; and one with a skull-shaped mask covering his entire face.

Complete Homunculi
Complete homunculi are those which most closely resemble the story of Paracelsus; they were created through blood sacrifice and born possessing all knowledge, but were trapped in the flask in which they were created. They are essentially the universe itself, concentrated into a single location and given the ability of human speech; as such, they can see the past, present, and future as well as control space and time as easily as one might move one's limbs.

While multiple people in the series refer to the sixteenth century alchemist Paracelsus as the originator of this concept, Ronny Schiatto was created in 300 B.C., so it seems that the method of creation was known prior to Paracelsus's experiments.

Complete homunculi have the option of escaping their flasks by forfeiting some of their knowledge or power, thus becoming incomplete. Most of them choose to forfeit some of their influence over the universe and retain their knowledge of the future; however, Ronny Schiatto made the opposite choice, to forfeit his knowledge of the future and retain the ability to interfere with the laws of reality.
 * Homunculi of this type include Ronny Schiatto and Rosetta. A number of other homunculi of this sort have been created (potentially including one by Paracelsus), but they have not directly affected the events of Baccano!'s story.