Born in 1492, Katherine was the second oldest of her siblings. Like her father, Gideon, Katherine was fascinated by science and technology. She was always close to her younger brother, Thomas Cahill. When she was little, she and Thomas spent endless hours dreaming of going on great adventures. In 1507, both she and Thomas believed Luke was responsible for the fire that killed their father, so they traveled to Portugal together to protect their Clues from their brother. While there, Thomas met his first love, a woman named Louisa. At his insistence, he and Katherine stayed in Portugal for three years.
One night, an angry Katherine, believing that Thomas chose love over duty, ran away from her brother with his Clue and escaped to Egypt. Ever since then, Thomas and his descendants have hated Katherine and her descendants. She then became one of the world's first Egyptologists, learning how to read and write Hieroglyphics even before the Rosetta Stone was found. There she hid a Clue for her descendants to find. According to the back of her card, she wrote that Luke was "sneakier than a serpent, and even more deadly;" that Jane was "too young, too fanciful to make it on her own;" that Thomas "ignored his duty, my greatest disappointment;" that her father "valued his secrets more than his family;" and that her mother, Olivia, was "blinded by love and crippled by loyalty". Katherine died at an unknown point of her life in the Tutankhamun Statue in Egypt while hiding a Clue.
"Ingenuity above all"
- Gideon Cahill - Father (deceased)
- Olivia Cahill - Mother (deceased)
- Luke Cahill - Brother (deceased)
- Thomas Cahill - Brother (deceased)
- Jane Cahill - Sister (deceased)
- Madeleine Cahill - Sister (deceased)
- Madeleine the Matriarch - grandmother (deceased)
- Toyotomi Hideyoshi - Nephew (deceased)
- Louisa - Sister-in-law (deceased)
- William Shakespeare - Great Nephew (deceased)
- Winthrop Cahill - Nephew (deceased)
- Many other relatives and descendants
As founder of the Ekaterina Branch, Katherine loved technology and the sciences. Bae Oh, former Ekaterina leader said that she was the "queen of ingenuity". She was courageous enough to travel the world alone, which was dangerous for a woman even in her time. She always seemed to prepare for the future, leaving them a Clue in the Sakhet statues, and in her diary for her descendants. Katherine was also extremely clever, managing to crack the code of Hieroglyphs 200 years before the Rosetta Stone was found.
When Dan Cahill found her portrait, she is described as a young woman with short blonde hair and intelligent eyes, wearing a brown dress. Unlike most girls at the time, she wore breeches rather than dresses. She is, however, shown wearing a dress on both of her cards. In Vespers Rising, when she was still fifteen, Gideon described her with short dark hair and dark eyes, and wore a frock and breeches like a boy.
A portrait of Katherine Cahill was found along with the pictures of her siblings in Paris.
It is revealed that Katherine went to Egypt to hide one of her Clues, Myrrh. She bought three Sakhet statues, modified them to hold secrets to the Clue, and hired grave robbers to hide them in tombs of queens.
Her tombstone was on Cahill Island.
In Vespers Rising, Katherine was seen by Gideon taking apart a globe, which was an ancient family heirloom; this may have been the Armillary Sphere. He did not punish her, as he was surprised she hadn't taken it apart years ago. In the prequel, Thomas looks up to her, Luke taunts her, and Jane has very little contact with her.
Katherine was skilled in technology and mechanics, which is passed on the Ekats, who are talented in math, science, etc.
- On Katherine's notebook, there is the alchemical symbol for Phosphorus, which is one of her Clues.
- Katherine was as smart as her father, being the only one in her family besides her father to outsmart Damien Vesper.
- She is holding an Armillary Sphere, according to Card 99 which may be due to her interest in astronomy.
- Her name means Ekaterina in Latin, which is likely why she named her branch Ekaterina.