She makes Bert's acquaintance when the two of them order the same plain, flat, soda waters just moments from one another. They make quick friends and soon discover they share many of the same interests: marching band music, oatmeal, and collecting paper clips and bottle caps.
They even keep the same pets: Bertha has a pet pigeon named Francine who she introduces to Bert's pet pigeon Gus.
Bertha can be seen in the large crowd of Muppets in the "Rainbow Connection" finale in The Muppet Movie.
Through the use of incidental music, a romantic relationship between Bertha and Bert is implied. Bert once sang about an unnamed girlfriend whose features would fall apart every time she saw him in "I Want to Hold Your Ear".