Frillish is a jellyfish-like Pokémon resembling young royalty. Regardless of gender, it has a round head with a crown-like tuft, and five tentacle-like appendages: two serving as arms and three as the lower half of its body. It has oval eyes with white pupils, and a collar around its neck. It has invisible poison spikes on its appendages. As a male Frillish, it is blue, with smooth appendages and a triangular crown. It has red eyes with blue sclerae and a straight eyelash on each eye, its collar is wavy, and it appears to be frowning. As a female Frillish, it is pink, with wavy appendages and a round crown. It has blue eyes with red sclerae and a curled eyelash on each eye, its collar is frilled, and it appears to be smiling.
Frillish is a hadopelagic Pokémon which lives in dens about 5 miles (8 kilometers) under the sea surface. Because of this, it can cope with extremely dark and highly pressurized sea conditions. It is carnivorous; it gently floats around in waves until it can strike its prey. It makes its victim numb with the use of its poison spikes and takes it to its deep den.
A male Frillish debuted in A Rival Battle for Club Champ!, under the ownership of Trip. He used Frillish in his match against Ash, and he was able to defeat both Pidove and Snivy, despite having a disadvantage against the latter in both type and gender.
Both male and female Frillish appeared in The Pokémon Harbor Patrol!, under the ownership of Halsey. They were used to help rescue people from fires, and then to help put out the fires alongside Dewott.
A Team Plasma Grunt used a female Frillish in a flashback in Unforgettable Memories. She fought against Hugh's Purrloin and Trapinch after Hugh refused to listen to them when they convinced him that he was causing pain to his Pokémon. Frillish was able to weaken Purrloin, forcing Hugh's sister recall it to its Poké Ball, which was then taken by two Grunts afterwards.
Frillish and its evolved form were most likely inspired by the Nurarihyon, a jellyfishyōkai that is either shown as a demonized jellyfish or as a jellyfish-like man who wears refined, sometimes regal, clothing. It is also possible that they was inspired by the Kurage no Hinotama, a type of jellyfish-like will-o'-the-wisp. Its design is inspired by the jellyfish and Victorian-style monarch attire: the male form's appearance based on a prince's while the female's is based on a princess'. The moon jellyfish has inspired Frillish in being a ghost type. The two larger arms Frillish possesses are more similar to those of squids than the tentacles of real jellyfish. Frillish and its evolution also have traits in common with sea monsters like the Kraken, most notably their rumored penchant for destroying ships and feeding on the crew.
Frillish may be a combination of frill and jellyfish.
Pururill may be a combination of プルプル purupuru (soft) and rill or frill.