Sonya Halliday

Professor Sonya Halliday is a lifelong friend of Arthur's and "well-known luminary in the fields of cryptography and complexity theory" who works at a university in California. She is "a tall, thin African-American woman who was probably in her mid-forties" who tends to be precisely dressed and groomed. In Root of Unity she makes a major mathematical breakthrough and is subsequently the target of several groups. She and Cas do some impressive math together, and Sonya wants to do more, but all of this leads to a shattering revelation that shifts the ground Cas stands on. The two of them continue corresponding after the book ends.