She is Abigail's granddaughter.Coordinator_Dawn and Ash were having a practice battle when Theresa came along and commented on Pikachu, wanting to borrow him for her aunt. She explained that Abigail's Pikachu, Sugar, ran away. After a bit of convincing, Ash finally agreed, as it was only for a few days.
When Theresa brought Pikachu to her aunt, she was overjoyed. When Abigail began having Pikachu lightly fry Aspear Berries off trees, both Ash and Theresa started getting worried if it was the right choice. When Abigail challenged Ash to a match, Theresa unwillingly refereed.
In the evening, Ash decided to tell Abigail about Pikachu. Surprisingly, Abigail revealed that she suspected that Pikachu may not have been Sugar. Theresa then confessed that she knew why Sugar ran away.
When Team Rocket came and tried to kidnap Pikachu, both Theresa and Abigail chased after Team Rocket's mecha. She was just as surprised as anyone when Sugar came back, now evolved into a Raichu.