Teodora Heterodyne (nee Vodenicharova) was the wife of Saturnus Heterodyne, mother of William and Barry Heterodyne, and thus grandmother of Agatha. To date she has only been mentioned in The Secret Blueprints (where she is not even named), and in a footnote in the novel Agatha H. and the Voice of the Castle, and so all information presented here should be considered tentative.
The two currently available sources of information agree that she was a brave and noble woman who was forced into marrying Saturnus on threat of harm to her family, and that it was she who was responsible for raising her two sons to be the heroes that they were. In the end, when Saturnus realized that his sons had forever rejected the traditional Heterodyne path, he announced his intention to kill them and start over. Teodora in return fatally poisoned him, resulting in her own death.
The novel offers a few additional facts along with her name: Saturnus genuinely loved her, even in the face of her dogged insistence to live and raise the boys away from Castle Heterodyne. Thanks to this attitude, and the death of Saturnus, it was the Castle who killed her. Her selfless actions led to her being canonized after her death, and declared patron saint to Those Who Fall Afoul Of Sparks.