In the past, they were known as Theons and their hosts as "avatars", until Eliza, while she was known as Neferu, and Sekhmet caused the world to fear such beings, leading to the discrimination that many face in the present.
Alex Ahad has stated that some artificial Parasites can be removed, depending on how they are attached to the body. 'The point of attachment can’t be removed without massive surgeries, but unlike normal parasites, artificial parasites can have removable parts'. While detached, a Parasite doesn't need food or sleep, though depending on the Parasite some can go into minimal forms (like some types of bacteria). Parasites take on a variety of forms, but can be classified as one of two groups: "regular" Parasites that occur naturally (and appear to predate the Skull Heart itself) and "Synthetic" Parasites created artificially to serve as weapons (mainly by the Anti-Skullgirl Labs). Synthetic Parasites are usually non-sentient.
List of Parasites
Note: Some of the below Parasites may be synthetic, but will be considered natural until official information states otherwise.
- Samson: A parasite that manifests itself as a head of living black hair on Filia's head. Easily distinguishable by his bright yellow eyes, nightmarish teeth, and brutish voice, complete with an equally intimidating appetite. While his primary concerns appear to solely encompass eating and dragging his host into peril, he has expressed care for Filia and serves as her primary means of defending herself. It is hinted that Filia was not Samson's first or only host, and that he has a natural reputation of putting his hosts in danger.
- Leviathan: A bone-like serpentine parasite that coils itself around Squigly's body (as well as through her head). As with Samson, Leviathan also has bright yellow eyes, but is otherwise the complete antithesis. Where Samson is crude in nature, Leviathan behaves like a gentleman, complete with a posh English accent. Leviathan also appears to be "living" independently from Squigly, making their bond closer to mutualism than parasitism. Before having Squigly as a host, he was 'attached' to her father and resided in his arm or shoulder area (also externally).
- Sekhmet: A bloodthirsty parasite inhabiting Eliza's skeleton. Sekhmet is responsible for keeping Eliza eternally young by absorbing the blood of others. It also enables Eliza to control her own blood as a weapon, and may be capable of operating independently of its host (possibly with the aid of Eliza's staff).
- Shamone: A parasite manifesting itself as a dark green head of living hair on Fukua's head. Shamone is seemingly a clone of Samson, created by Brain Drain along with Fukua. Shamone takes the exact appearance of Samson aside from being a dark green color instead of black. While Shamone shares many abilities with Samson, Shamone also has abilities of his own that Samson does not have, such as shooting projectiles.
- Muse: The remote parasite owned by Talesin. Muse has replaced Talesin's arms with her own as the price for remote operation. The arms she has given him allows him to play any instrument as well as curses Talesin's touch to turn people into instruments. She cares deeply for him and becomes very jealous of his womanizing tendencies. However, she either cannot or refuses to speak and thus, never voices her disdain. When her legs fold inward, she somewhat takes the appearance of a viola.
- Sagan: The Parasite of Annie, Girl of the Stars. It is a remote parasite that resembles a stuffed rabbit housing Annie's right eye in his mouth. Based off sketches by Alex, Sagan can be ridden as somewhat of a flying scooter, with his ears extending into long handlebars.
- Khronos: Aeon's Parasite, resembling a golden snake wrapped around her waist. It presumably has a hand in Aeon's time-based powers, but it is unclear how much of Aeon's powers are derived from Khronos and how much are her own.
- Ab-baddon: The Parasite(s) controlled by Venus. While not much is known about them, they appear as giant serpents bursting from the mouths around Venus's torso. They may be multiple entities or a single, multi-headed being - or, given Venus' hinted true nature, a part of her real body that simply appears as a Parasite.
Synthetic Parasites are artifical parasites created by man. Lab 8 specializes in such development, however since they are not true natural parasites, most of them do not have any sentience. Currently, the Avery Unit (and by extension, the rest of Peacock's gang) is the only man-made Parasite shown to have independent free will and intelligence.
- Argus System: A Synthetic Parasite created in ASG Lab 8 that replaces Peacock's arms and gives her her namesake. Appears as two slender metallic "arms" with metallic "eyes" placed at intervals on each arm. In addition to providing 360-degree sight, the "eyes" are able to fire Z-rays fine-tuned to combat the Skullgirl. Peacock is able to remove this parasite, akin to an action figure.
- Avery: Peacock's other Synthetic Parasite is a remote parasite appearing as a small peacock garbed similarly to its partner. Avery lives inside Peacock's top hat, often appearing to aid her in combat. Its primary purpose is to provide a spacial link between Peacock and Lab 8's weapons cache for her usage. There is a limit to what can be summoned though as Peacock is unable to literally produce anything out of thin air, since they are modified arsenals. Since her powers are still growing however, she might be able to summon more things in the future.
- Gae Bolga Matrix: A Synthetic Parasite developed in ASG Lab Zero for Painwheel's use. Its true form is unknown, but it resides in Painwheel's bloodstream and enables her to release large, organic needle-like projections from various parts of her body. This occurs as the parasite takes the form of a metal when it leaves her body. While doing this would normally kill the user, Painwheel's extensive training and Skullgirl blood in her allows her to use it quite extensively.
- Buer Drive: Painwheel's second Synthetic Parasite is a segmented spine-like cord which commands the helicopter blades mounted on Painwheel's back. Through it, she controls the motions of her blades as like a limb, enabling flight with it or selectively clash down its individual blades. It is grafted into the center of her spine, and is branched off from the blade by an appendage made up of eight segments connected by fibers underneath. This appendage is flexible and somewhat stretchable. Buer Drive has removable parts that can be kept in a cello case, though this has not been demonstrated in-game.
- Taranis Generator: Leduc's Synthetic Parasite. It assists Leduc by generating and conducting electricity.
- Beezle Bomb: Hive's Synthetic Parasite. Its default shape is a set of interlocking hexagons wrapped around her arms, but can be shifted into various configurations such as a lance or shield. She’d also be able to launch them as bee-shaped missiles and bombs.
- Salamander Shell: A Synthetic Parasite used by Ileum. Its capabilities are as of yet unknown, but it may be the source of Ileum's metallic drills or a binding to hold hold her body together.
- It is possible to remove a parasite from its host, however, Alex Ahad stated that if a parasite is removed from its host, both will die in the process. Despite this, Leviathan is said to have been passed down through different generations of the Contiello family, and survived his separation from Squigly's father long enough to join with her, although he appeared to have died along with her shortly afterwards.
- Parasites can change hosts if their current host dies, gets too old, or cannot handle them anymore. The act of changing hosts would kill the former one.
- Synthetic Parasites can be removed without killing the host.
- Parasites can not survive very long without a host. But their abilities to survive outside a host vary. They do not necessarily need food or sleep when detached from a host.
- Some Parasites can go into “minimal” states like a bacteria going into a cyst form, but it won’t last them forever.
- A remote parasite needs a part of the host body in order to be bound to the host, for example Taliesin's hands or Annie's eye.
- Parasites can greatly vary in age, some are ancient while others are relatively new. New ones can be created. They can stay alive forever as long as they have a host.
- Creative director, Alex Ahad, stated that all the Parasites and Living Weapons have a related, unrevealed, origin. It is confirmed that the mysterious element Theonite is the source of both their powers.
- Soul Contamination is a mental corruption that occurs in people possessing multiple (synthetic or natural) parasites. It could potentially kill the person and is very dangerous. In Peacock's story mode Hive notes that it must have took great difficulty for both of her parasites to be implemented without causing soul contamination. At the beginning of Painwheel's story mode, it is noted that soul contamination had to be stabilized before implementing Painwheel's control mechanism and guaranteeing that the project was an overall success.
- When a Parasite host becomes the Skullgirl, the Parasite too is likewise corrupted. They could lose their individuality as well. This process also depends on the wish of the host as well.
- Parasites, like Living Weapons, manifest their powers differently from host to host.