Theresa is a minor character from Pokémon: Diamond and Pearl.
Season 10: Diamond and Pearl
Ash Ketchum, Dawn and Brock met Theresa when she saw Ash's Pikachu on Ash's shoulder. Theresa requested Ash if her aunt, Abigail, could use Pikachu temporarily while Pikachu pretented like it was Sugar. Theresa explained that Sugar, Abigail's Pikachu, left her and never came back and that a Pikachu was needed to make the perfect cake. Ash agreed on her request and Theresa took the heroes to her aunt. Pikachu did a great job but Theresa was worried that Pikachu couldn't left Abigail because Abigail thought it was Sugar. Team Rocket attacked and the real Sugar came out by firing a Thunderbolt, protecting Abigail from Team Rocket. Sugar was revealed to have evolved into Raichu. After Team Rocket blasted off, Abigail explained that she already knew Ash's Pikachu wasn't hers from the start, surprising Theresa and the heroes. Theresa and Abigail later bid farewell to the heroes when they left for their journey.