Sonia Shaflnarz comes from a family part of the Sicilian mafia. She moves to Japan in order to get revenge against the Sanzenin family, because her father died due to the family, though not directly. In "Extra- Radical dreamers" of chapter 41, her father hid in a trash can to assassinate Nagi on Mykonos island, but Hayate prevented it and defeated him. After that, he quit the mafia and went to Japan to become a cook. He then died because he didn't know that there was poison in blowfish. She impersonates Sister Fortesia, a sister from the Church of Alexander Mark and the Tiger's Pit for Butlers for the purpose of approaching Hayate and the Sanzenin family. She speaks fluent Japanese, but she uses harsh words and blames it on the fact that she doesn't know Japanese very well. Sonia has unbelievable strength and fighting ability and uses tonfas as weapons.
In volume 8, it is revealed that she has some feelings for Wataru, going to his DVD rental store and growing extremely jealous when she sees him expressing concern over Saki. However, when Saki is kidnapped, she uses the situation to get Wataru to kiss her on the cheek in exchange for Saki's rescue. By the time Hayate (who was on the scene at the time of Saki's kidnap) reaches the kidnappers on his bike, he finds that the kidnappers are already tied up, their car impaled by a telephone pole in the shape of a cross, and Saki looking confused and unhurt on the side.