Nyarth, also called Meowth, is a small feline Pokémon with cream-colored fur and brown markings on its hind paws and tail tip. Its ovoid head features wide eyes with slit pupils, two pointed upper teeth, four prominent whiskers, and a distinctive gold koban coin embedded in the forehead. Black ears with brown interiors and additional long whiskers complete its features. This Pokémon can retract its claws for silent movement, typically walks on all fours, but also walks bipedally. Nocturnal in nature, Nyarth exhibits a strong attraction to round, shiny objects and generates coins using its signature Pay Day move, often sleeping among collected treasures.

Distinguished by rare human-like intelligence and self-taught human speech, Nyarth commonly finishes sentences with "Nyā". Within Team Rocket, it collaborates with Jessie and James in schemes to capture rare Pokémon, especially Ash Ketchum's Pikachu. Nyarth frequently devises elaborate plans, though they are typically thwarted. One notable scheme involved feigning defection to infiltrate the Raimon City Pokémon Center. After gaining Ash's trust, it bypassed security to steal vault Pokémon before betraying the group, though the plan ultimately failed.

In "Pokémon 3D Adventure: Mew o Sagase!", Nyarth partnered with Pikachu on a quest to locate the Mythical Pokémon Mew, cooperating across environments like the sea, sky, and forests—a rare departure from their usual adversarial dynamic.

The species displays regional variations:
- Alolan Meowth has light blue-gray fur with whitish-gray markings, curved whiskers, and silvery eyes. Originally royal gifts in Alola, this variant developed a prideful, impulsive, and cunning disposition from luxurious living. It reacts hysterically if its coin or pride is challenged and proves difficult to train.
- Galarian Meowth possesses a shaggy greyish-brown coat resembling a beard, yellow eyes, and a black hardened forehead coin. Years alongside seafarers partially turned its body to iron. This variant evolves into Perrserker instead of Persian and displays antagonism toward Kantonian Meowth.

Some Kantonian Nyarth can Gigantamax, growing significantly larger with an elongated body and enhanced tail. The forehead coin gains a symbol and emits Gigantamax power, intensifying its desire for shiny objects to destructive levels; this coin symbol is theorized to hold Dynamax-related secrets.

Kantonian Nyarth evolves into Persian, while Alolan and Galarian variants evolve into Alolan Persian and Perrserker, respectively.