Theresa Brisby is the eldest child of Jonathan Brisby and Mrs Brisby (her siblings being Martin, Timmy and Cynthia. Due to her being the eldest, she felt compelled to act responsible and mature, though this sometimes made her quite short-sighted (such as in her refusal to contradict Auntie Shrew even at her stupidest).
The Secret of NIMHEdit
Theresa was only occasionally involved in the events that occurred in 1982 and led to her mother finding out the truth about the Rats of NIMH and to the Rats' subsequent departure for Thorn Valley. She joined her siblings in 'interrogating' a bound Jeremy while Mrs Brisby visited Nicodemus, a process which did not lead to anything of note. Also like her siblings, was inside the Brisby House when it sunk, and nearly drowned in the mud before Mrs Brisby rescued them.
Little is known about Theresa's later life. Like Martin and Cynthia, she stayed behind on the Fitzgibbon Farm (presumably helping her rapidly aging mother around the home) when her brother Timmy moved to Thorn Valley. After maturing, Theresa had stopped wearing her trademark large pink bow.