From Bulbapedia, the community-driven Pokémon encyclopedia.
Sierra (Japanese: ステラ Stella) is a recurring character in the Pokémon anime who was featured in A Restoration Confrontation! Part 1 and A Restoration Confrontation! Part 2. She is a Pokémon archeologist and childhood friend of Ferris. Though she was his friend, she was often hard on him and referred to him as an amateur archeologist who only made trouble for her.
Sierra when she was younger
As a child she, Ferris, and a few of their friends often played in the off limit area of Twist Mountain and were all part of a detective group. Each wore a special pendant that symbolized their membership. One day Ferris got lost while they played and they eventually found him unconscious which ultimately caused Sierra to be more respectful of the rules.
For years she did not believe Ferris's story of traveling through time and meeting a real life Tirtouga. She was convinced that it was only a dream he had. Once Team Rocket stole the Cover Fossil Ferris had found and he and Ash went after them alone, Sierra followed behind them in a truck along with Iris and Cilan. After they had escaped Team Rocket's helicopter, she picked them up and drove away from the scene.
Sierra would eventually believe Ferris's story after meeting Tirtouga and seeing the Time Gate for herself.