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, 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, but in the anime it is shown to smile rarely. As a female, 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. Its arms are filled with tens of thousands of poisonous stingers. There is a legend where Frillish are said to have been created from transformed residents of a sunken ancient city. Frillish preys on Magikarp and Lanturn. Female Frillish have slightly longer stingers on their arms, while male Frillish have bigger collars.
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.
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 Jellicent were originally pure Water-type. The Ghost type was added to their typing for balancing reasons, with Ken Sugimori noting that the designer for the Frillish line was disappointed that they ended up becoming scary Pokémon.
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 were inspired by the Kurage no Hinotama, a type of jellyfish-like will-o'-the-wisp. Frillish and its evolved form may also be inspired by real-life jellyfish such as the Stygiomedusa, also known as the Phantom Jelly, a very rarely seen deep sea jellyfish with fabric-like arms. The two larger arms Frillish possesses are more similar to those of squids than the tentacles of real jellyfish, however.
Frillish's design is inspired by the image of princes and princesses, perhaps specifically of the Victorian-style monarch attire. Frillish and its evolved form 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 (onomatopoeia for jiggling) and frill.