Castform is an artificial Pokémon that changes appearance and attitude based on the weather. Its body is made of cells exactly like those of water molecules, causing its cellular structure to be altered by temperature and humidity. It uses this ability to protect its tiny body. In its Normal Form, it resembles a plain, gray cloud with a single wisp extending from the top of its head. When in its Sunny Form, its lower half becomes pure white, while the head turns orange has develops small orange spheres around it. This gives it the overall appearance of the sun above a small cloud. While in its Rain Form, it will have a dark gray lower half resembling a storm cloud. The head will be blue and resemble a raindrop. In its Snowy Form, its head turns purplish-blue. The rest of the body changes into the shape of a mint green, cumulonimbus cloud. Regardless of the form it takes, it will always have a large, spherical head and a lighter colored mask-like marking over its eyes. Aside from its various forms, Castform is able to utilize weather powers in the form of its previously signature move, Weather Ball. Despite its manmade origins, it can be found in grassland areas.
A Castform, nicknamed Popo, belongs to Ruby in Pokémon Adventures. Previously, she belonged to Mr. Stone, who was injured in an attack by Team Aqua. Popo was then given to Gabby and Ty to be cared for while he was being healed. She later decided to travel with Ruby when he began traveling with the reporters because she was lonely. Until he was reunited with Rara, Popo was Ruby's Smart Contest Pokémon.
A Castform makes an appearance in Descent Of The Weather God!! of Pokémon Ruby-Sapphire. In that chapter, it showed the ability of transforming into many forms depending on the current weather.
Castform borrows the power of nature to transform itself into guises of the sun, rain clouds, and snow clouds. This Pokémon's feelings change with the weather.
Castform's weather-based transformations cannot be viewed in the forms section of the Pokémon Diamond, Pearl, and Platinum Pokédex, nor do they appear on the field in Pokémon HeartGold and SoulSilver. They do, however, appear in the forms section of the HeartGold and SoulSilver Pokédex.
Castform's forms will appear in the forms section of the HeartGold and SoulSilver Pokédex upon obtaining one, even if never transformed.
Unlike most Pokémon possessing alternate forms, Castform's alternate forms have no Shiny variations of their own. Only Castform's Normal form has a Shiny sprite.
It appears to be intended to resemble and act like a living cloud. Its ability to change shape and type depending on the weather condition makes similar to a weather balloon. It's also similar both in concept and design to a teru teru bōzu, a Japanese charm doll used to wish for good weather. Its forms, especially their types, may have originated from the English saying "Rain, Hail or Shine". Also, as Castform is said to be composed of molecules similar to water, its shape of a large orb with two smaller resembles an H2O molecule.
Name origin
Castform may be a combination of forecast (the prediction of weather) and transform or form (referring to its ability to change form depending on the weather). The German and French names reflect this.
Powalen may be a combination of ポワン powan (onomatopoeia of a sudden transformation, similar to "poof" in English) and 変わる kawaru (to change or transform).