|First Appears In:||Vespers Rising|
|Last Appears In:||Shatterproof|
Erasmus Yilmaz was an Ekaterina and Madrigal, and at first Dan thought he was a very angry biker and was going to terrorize Fiske Cahill in front of a coffee shop. Amy and Dan attacked him with a windshield wiper. He was nicknamed by Dan as "Big Scary Harley Dude." It is also known that he "has a Vesper database in his head".
- Katherine Cahill - x22 Grandmother (deceased)
- Thomas Cahill - x22 Granduncle (deceased)
- Jane Cahill - x22 Grandaunt (deceased)
- Madeleine Cahill - Great x22 Grandaunt (deceased)
- Gideon Cahill - Great x23 Grandfather (deceased)
- Olivia Cahill - Great x23 Grandmother (deceased)
- Madeleine the Matriarch - Great x26 Grandmother (deceased)
He usually wears leather leggings, a leather jacket, and leather boots like a biker. His hair is a mass of black curls and he has gray eyes. He appears to be good friends with Fiske Cahill and William McIntyre.
He, Nellie, and Fiske told Amy and Dan about the Vespers. Fiske said that he has a Vesper database in his head. He also comes to help Amy and Dan when they are being chased by the Vespers. He was able to ski through along with Fiske.
He meets Amy and Dan to discuss that their father was a Vesper. In the end of the book when William McIntyre dies he later cries over William McIntyre's dead body.
He was a suspect for Vesper Five but proven not to be when he sunk Vesper Five's ship in the Bermuda Triangle. It is also found out that he was an Ekaterina.
Erasmus breaks into a Vesper-safe house and battles Luna Amato. He drops his weapon and turns toward Milos Vanek. They spoke and argued with Luna and Milos. In that split second Luna grabs his weapon and impales Erasmus through the heart. Jonah and Hamilton rush in and Jonah shoots Luna, killing her. The dying Erasmus gives Jonah and Hamilton a flash drive and gives an unfinished warning about the Vespers before he dies.
- He once rode his motorcycle from Paris to China.
- He was born into a family of Ekaterinas but was recruited by the Madrigals as a teenager.
- He sank Vesper Five's ship in the Bermuda Triangle.
- In the sneak peak of Shatterproof, it said that the Vespers murdered his parents. It also said he thinks Luna Amato murdered William McIntyre.
- He can speak nine different languages.