• English / United States: Sierra
• Japanese / Japan: ステラ
• Japanese (Romanized) / Japan: Stella
• Japanese (TL) / Japan: Stella

Sierra works in the mines of Twist Mountain. She has been friends with Ferris since childhood and they often used to play together on Twist Mountain where they formed the Twist Mountain Detective Group and conducted missions together. They even entered an off-limits zone once, which caused them to get scolded pretty badly but Ferris met a real Protoga after he fell through a space-time distortion and was sent back to the ancient world.

She didn't believe Ferris's outlandish claims and got mad at him for loosing his Twist Mountain Detective Group pendant. Ferris spent many years searching the mines of the mountain looking for ancient Pokémon fossils when he made a huge discovery the day Ash Ketchum and his friends where traveling through. Until the day Ferris found a Protoga fossil, Sierra believed the rumors that there were no fossils on Twist Mountain.

Sierra is quite bossy and doesn't take well to people saying things without proof. It is unclear if she has romantic feelings for Ferris or just considers him to be a close friend. With the help of Ash Ketchum and his friends, Abagoura was returned to its own time with its friends and Ferris was able to prove to Sierra finally what he had said for years was actually true.