Bertha Jorkins was born c. 1958. She was two years ahead of Sirius, James, and their friends at Hogwarts. Bertha was a gossip and didn't seem to have a lot of social skills--once she spotted someone "kissing Florence behind the greenhouses" and didn't have the sense just to ignore it, which resulted in that person casting a hex on her. Harry saw her in Dumbledore's Pensieve at about age sixteen and she appeared as a plump, scowling girl who complained about ill treatment from other students.

She went to work for the Ministry of Magic after Hogwarts. Some were of the opinion that she was shunted from department to department. Bertha ended up working for the Department of Magical Games and Sports. She had a reputation for being somewhat scatterbrained, so much so that when she turned up missing on a vacation to Albania the summer before the Triwizard Tournament, most people assumed she'd simply lost track of time and would come wandering back in a week or two.

At some point before that fateful summer of 1994, Bertha had gone to the Crouch residence with some work for Barty Crouch Sr. While there, she had caught sight of Barty Crouch Jr. and had realized at least part of the truth: that the boy had escaped Azkaban and that he was being hidden in the house. Crouch Sr. had then placed a strong Memory Charm on Bertha to make her forget what she'd seen. It was this Memory Charm that caused her to be so forgetful. 

During her holiday in Albania, Bertha didn't simply lose track of time and place. The truth is that she had met Peter Pettigrew at an inn there and he had led her to Voldemort. The Dark Lord had used powerful magic to break the Memory Charm on Bertha and thereby discovered that plans for the Triwizard Tournament and the fact that his faithful servant Barty Crouch Jr. was still alive. The magic Voldemort used to extract this information from Bertha was so powerful and invasive that she was left "damaged," so he killed her.

The "shade" of Bertha came out of Voldemort's wand during the Priori Incantatem event. Bertha knew what was going on and who Harry was, and she urged him not to let go of the wand. She joined the other "shades" of Voldemort's victims in encouraging Harry and harassing Voldemort.