Nihilego

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 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 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, 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. In its home dimension, the anime shows that Nihilego can control its environment.

As shown with Lusamine, Nihilego is capable of physically merging with a 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. 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 the anime showsthat Nihilego can willingly end this merger and it may be disrupted in other ways as well, the lingering toxins and the shock of the separation can leave the host in critical condition, both physically and psychologically.