Nihilego is an Ultra Beast which resembles a jellyfish. It is mostly white with a faint blue tint and has tentacles shaped like the hair, dress, and legs of a human girl. The top of its head is a bell that resembles a sun hat. The bell is transparent save for six pointed stars around the crown, and the brim has a blue edge. The ends of the "hair" fly out, resembling arms, and the outer tentacles of the "dress" have two bubbles inside. Nihilego is composed of an oddly flexible glass that varies in opacity throughout its body, and is apparently capable of shapeshifting - something it is said to frequently do.
Nihilego is a parasitic organism and, by latching onto one's head, is capable of injecting a neurotoxin that subtly alters the thoughts and actions of any human or Pokémon that becomes its host. This neurotoxin stimulates intense feelings of excitement while causing a loss of inhibitions, which acts to forcibly bring out any latent potential within the host while wearing away at their minds.
Nihilego's exact levels of intelligence are unknown, with conflicting degrees of apparent sentience, but according to the Aether Foundation's research it seems to act mainly on a self-preservation instinct. It has been observed behaving like a young girl, though the exact reasons for why it does so are unknown.
Concept art of Nihilego's Ultra Space reveals that some of the crystalline structures in the dimension are in fact polyps and strobila, revealing that Nihilego reproduces in a similar way to real-life jellyfish rather than the conventional method of Pokémon laying and hatching from eggs. These polyps slowly grow when planted into the ground and blossom into strobila, which then split into multiple free-floating Nihilego ephyra that eventually mature into the medusa-like form.
Spoiler warning: this article may contain major plot or ending details.
Nihilego fused with Lusamine
As shown with Lusamine, Nihilego is capable of physically merging with a host. Concept art shows Nihilego absorbing Z-Crystals and growing in size before enveloping the host. In this "merged" state, the bell loses its stars, developing several iridescent spots across its surface; the host's body is visible within the bell, with their legs dangling beneath it - Lusamine's hair changes from blonde to dark purple in the process. Its eight tentacles become long and flattened in appearance, and what appear to be eyeballs with colored diamond-shaped "pupils" cover their surface, along with smaller multi-colored dots. Four of the tentacles are connected to the "hat" via frilly white structures; these tentacles have four spines protruding from them that can be used similarly to hands. The remaining four tentacles each terminate in a single smaller spine. While this merger can be disrupted, the lingering toxins and the shock of the separation are likely to leave the host in critical condition, both physically and psychologically.
Nihilego is the only genderless Poison-type Pokémon.
Nihilego is the only Rock-type Pokémon that cannot learn the move Rock Slide.
Origin
Nihilego may be inspired by a jellyfish, especially Aurelia aurita. It may also draw inspiration from Myxozoa, a class of microscopic Cnidarian parasites, particularly the Myxobolus cerebralis that are known to cause the infectious "whirling disease" in various fish species. The disease causes the fish to develop bone and neurological abnormalities that cause them to swim in a corkscrew-like "whirling" motion. Visually, and due to its status as an Ultra Beast, it may derive inspiration from Phyllorhiza punctata, an invasive species in the waters surrounding Hawaii.
The merged form of Lusamine and Nihilego may be based on Turritopsis dohrnii, a species of jellyfish that is biologically immortal and can revert to a young, immature state if it is too old or stressed.
Name origin
Nihilego may be a combination of nihil (Latin for indefinite nothing) and ego (Latin for I), which together roughly translates to "I am nothing" or "I have nothing"; or the part of the psychic apparatus ego, which is the rational decision making part of the psyche) and together with "nihil" refers to how Nihilego has no ego, and only has an id (the survival and instinctual part of the psyche), which obviously relates to Nihilego. The former may reflect Nihilego's victims' lack of will outside their focus on Nihilego, while the latter may represent Nihilego's role in dominating the desires of its victims.
Uturoid may be a combination of 虚ろ utsuro (void), void, and id. It also might refer to 鬱 utsu (depression), 移ろい utsuroi (the act of moving from host to host), and android or the suffix -oid (meaning "of similar form").
In other languages
Language
Title
Meaning
Japanese
ウツロイド Uturoid
From 虚ろ utsuro, void, id, and possibly the suffix -oid (meaning "of similar form")