Mutou

Ranger Mason is a character of the day who appeared in Mother of All Battles (EP266).

Mason is one of the Pokémon Rangers in charge of protecting a Pokémon preserve on Mt. Silver. It is the same preserve where the Pokémon Poacher Brothers kidnapped Ash's Larvitar when it was just an Egg and severely wounded the Tyranitar that was Larvitar's mother.

Mason made it her mission to stop the poachers from hurting any more Pokémon. With the help of Ash and his friends, Mason was able to track down the poachers and capture them once and for all. Larvitar was successfully reunited with its mother. Mason thanked Ash and his friends for taking care of Larvitar and helping her capture the brothers, and they continued their journey to the Silver Conference.

In Tie One On! (EP270), Professor Elm called Ash with an update from Ranger Mason, saying Larvitar and its mother were doing well.

(Source: Bulbapedia)