Castform

Castform is the weather Pokémon. It changes types and appearance depending on whether the weather is normal, sunny, raining or hailing. Because of its unique abilities with weather, it got the attention of Team Magma/Aqua in Pokémon RSE.

Its name is a combination of "forecast" and "transform", being its body changes with the weather. The name could also derive from "overcast", as rain is a type of weather Castform can bring about.

Characteristics
Castform's appearance is a small, grey, cloud-like creature with a rounded body that can transform. It has an odd antenna on its head used for an unknown purpose. Castform appears to be wearing goggles, or a print to resemble such. It might be used like a satellite dish to sense upcoming weather changes. Castform has a distinction from other Pokémon in the fact that depending on the weather, it can transform into different shapes, described below.

Like Porygon, Castform is one of few human-made "artificial" Pokémon. It was created on purpose by the researchers of the fictional Weather Institute in Hoenn region as an experiment in weather forecast and manipulation. However, in an Arceus legend, a story states that it created a single speck of time and space, and then left it on Earth for future living things to create yet another thing that controls climate, which could be a reference to Kyogre and Groudon.

Castform's cellular structure is closely related to the weather conditions, meaning that Castform can both affect and be affected by the weather. Depending on the current weather and climate, Castform's emotions and even body are altered. In this way, Castform tries to adapt itself to multiple environments.

Castform is a rather unique pokémon. It is the only Pokémon with the ability forecast and one of two with the move Weather Ball. Both are affected by the Weather, and weather can be changed either by Castform or an opposing Pokémon. Castform can naturally learn three weather moves which changes the typing of Castform and weather Ball: Sunny Day (fire), Rain Dance (water), and Hail (ice). It will become the type of the move, (having the same weaknesses and resistances), and Weather Ball will do that type damage. It can also learn, by using a TM, Sandstorm. While Castform will not transform into a Rock type during a sandstorm, Weather Ball will do rock-type damage. In Pokémon XD: Gale of Darkness, if Shadow Sky is used, Castform doesn't change, but its Weather Ball attack becomes Dark type, a strategy used by Snattle.

Availability
Castform is available in Ruby, Sapphire, Emerald, Diamond, and Pearl. Only one Castform can be acquired in Pokémon Ruby, Sapphire, and Emerald (although additional copies can be bred normally with Ditto), and it is awarded to the player after he or she successfully defends the Weather Institute against Team Aqua or Team Magma, depending on game version. In Diamond and Pearl, wild Castform can be found in the Trophy Garden.

Castform's man-made nature is reflected in its uniform stats, all of which have the average value of 70. Its natural moveset is entirely geared towards manipulating the weather conditions. Castform's knowledge of the weather changing techniques Sunny Day, Hail and Rain Dance, together with the Weather Ball attack and the Forecast ability (both traits unique to Castform) encourage a very particular strategy in battle:

The Weather Ball attack has a mediocre base power of 50, but it becomes much more powerful under a weather condition, receiving the following bonuses:


 * Weather Ball's power doubles under a weather condition (Sunny Day, Rain Dance or Hail but not Sandstorm).
 * Weather Ball's elemental type changes to Fire for Sunny Day, Water for Rain Dance and Ice for Hail. Since Castform's type changes accordingly, this results in a 50% same-type attack bonus. (Weather Ball will become Rock type if Sandstorm is used, and in Pokémon XD Dark type if Shadow Sky is used, but Castform's type will not change.)
 * Sunny Day and Rain Dance increase the power of Fire and Water-type attacks respectively by 50%.

This results in a power as high as 225 (for Fire and Water) or 150 (for Ice). So, the most obvious strategy is to use the weather changing move whose type has an advantage against the opponent and then use Weather Ball.

Weather Ball also becomes even more powerful in Diamond & Pearl because of the new attack changes. Weather Ball now becomes a special attack so it depends on the Special Attack rating rather than the relying on the original Attack rating. Also, fog is a new factor in weather.

Although Weather Ball was exclusive to Castform in the third generation Pokémon games, Roserade, the evolved form of Roselia is the only other Pokémon capable of learning Weather Ball, although it can only do so by bringing it to the Move Relearner in Pastoria Town in Diamond and Pearl. By using rain dance with thunder and sunny day with solarbeam, Castform can be a useful special sweeper.

Anime
Castform was featured in episode #359, titled Unfair Weather Friends, in which operatives from both Team Aqua and Team Magma attacked the Weather Institute trying to steal data on Kyogre or Groudon, as well as weather-changing technology. It also appeared in another episode as a victim to be mistaken as Spoink's pearl.