Nihilego's exact bodily functions are unknown, however, it is known to be somewhat parasitic in the sense that it's much stronger when it has a host. Its also not known what exactly its tissue is made of, but according to several official sources, it's said to be made up of a glass-like material that is flexible like a rubber. As such, it is a genderless Pokémon. It's very perplexing, but more may be revealed in future games.
As implied by its title of Symbiont, Nihilego is specialized towards influencing other Pokémon to protect itself and its nests. It generates a neurotoxin that simultaneously gives those injected with it great power while reducing their inhibitions and sending them into an agitated, often violent state. The ultimate goal of this is for the infected Pokémon to attack and destroy any natural competitors for itself and the Nihilego. It is heavily implied that the violent behavior of other Ultra Beasts is due to acting under the influence of Nihilego venom.
As Nihilego is said to be the one who actually caused Lusamine to become evil, Nihilego is the first Pokémon without a trainer who is an antagonist.
It is also the first Pokémon to actually be fought as a trainer in the games after fusing with Lusamine.
Lillie's original appearance is based on Nihilego's appearance, as per request of her mother, Lusamine.
Despite being based on a jellyfish, it is weak to Water-type attacks.
Nihilego is the only Ultra Beast with a unique type combination that no other Ultra Beast or Pokémon has.
Nihilego is based on a jellyfish. Its mind-numbing poison may be based on the neurotoxin produced by the Portugese man-o-war.
Nihilego is a combination of "nihilist", a person who believes all life is meaningless and that there is no god, "ego", the mind space in which there is separation between consciousness and unconsciousness, and "geode", referring to its part Rock typing and its see-through appearance.