Ui's Castform, nicknamed Cassie
Castform is Ui's fourth Pokemon she caught prior to Full Moon's Full Pokemon Journey, she appear near the end of the first season and appeared again in the second seasone, she has a habit of changing forms depending on the weather.
Ui's Castform, Cassie, in its Rainy form
She can predict what weather is coming and alerts Ui in the process.
Ui's Castform, Cassie, in its Snowy form
In "What's In a Nickname?", she was officially nicknamed Cassie by Ui, and even nicknamed the other Pokemon she has owned.
Castform's known moves are: Weather Ball, Sunny Day, Hail, Rain Dance, and her ability is Forecast.
Ui's Castform, Cassie, in its Sunny form