Nihilego is an Ultra Beast which resembles a jellyfish. It is mostly white and has tentacles shaped like the hair, dress, and legs of a girl. On top of its head is an object that looks like a sun hat. This "hat" has 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 a parasitic organism, capable of injecting a neurotoxin that manipulates the thoughts and actions of the 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 at the expense of its mental stability. At the same time, it seems to subvert its host's personality and instincts in a manner that benefits Nihilego directly at the expense of the host and others. Its exact levels of intelligence are unknown, 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 this are uncertain.
As seen with Lusamine in Pokémon Sun and Moon, Nihilego is capable of physically possessing its hosts in addition to manipulating their minds. In this "merged" state, the "hat" loses its stars, developing several iridescent spots across its surface; the host's body is visible within the "hat", with their legs dangling beneath it. 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.
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 infect "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.
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 ego, as a reflection of its ability to enslave its hosts.
Utsuroido may be a combination of 虚ろ utsuro (void), void, and id,