"But she'll turn up. Poor old Bertha ... memory like a leaky cauldron and no sense of direction. Lost, you take my word for it. She'll wander back into the office sometime in October, thinking it's still July."
-- Ludo Bagman (GF7)
'Maybe she's changed since I knew her, but the Bertha I knew wasn't forgetful at all - quite the reverse. She was a bit dim, but she had an excellent memory for gossip. It used to get her into a lot of trouble, she never knew when to keep her mouth shut.'
-- Sirius Black (GF27)
Bertha Jorkins was a Ministry of Magic witch murdered by Voldemort.
A few years ahead of Sirius, James, and their friends at Hogwarts, Bertha Jorkins was gossipy and not especially bright (GF27). She was a plump, scowling girl, and tended to whine about ill treatment from other students (GF30).
After graduating Hogwarts, Bertha went to work for the Ministry of Magic, where she earned a reputation for being scatterbrained; she was moved around several different departments including International Magical Cooperation and Magical Games and Sports (GF5). When she went missing on a vacation to Albania in 1994, most people simply assumed she’d lost track of time and would come wandering back shortly (GF7).
Unfortunately for Bertha, she accidentally discovered that Barty Crouch Jr. was being hidden in the Crouch household; Crouch Sr. placed a strong Memory Charm on her to make her forget it. This was responsible for her memory becoming poorer (GF35).
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 the 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 (GF33) and used her death to turn the snake Nagini into a living Horcrux (BLC).
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 (GF34).
Other canon notes and references
Sirius told Harry in GF19 that Bertha was "a few years above" himself and James at Hogwarts, so she was at least two years older but no more than six.
Bertha doesn't seem to have been clever enough for Ravenclaw nor the type for Slytherin; given that Sirius knew her fairly well and that she was bold enough to confront Crouch senior, she may have been a Gryffindor. However, that is merely speculation. -- MLW