Arf was a former villainess in Magical Girl Lyrical Nanoha.
According to Sound Stage 2, she began her life as a pup for a wolf pack that lived in the mountains of Mid-Childs. She later contacted a contagious disease, and was cast out to ensure the health of the pack. When she was dying, Fate Testarossa comes along, and, feeling pity for her, begs Rinis to save her life. They later remove her soul, and merged it with a magical one, thus making her Fate's familiar. Even though she could have full freedom because of the contract, she chose to help protect Fate instead.
In episode four of the anime, she tries to help Fate find the Jewel Seeds in Uminori City. While fighting with Yuno, she tried to distract him while Fate was fighting Nanoha. She has been trying to tell Fate about her mother's treatment of her, but she was stubborn and would not listen. In episode five, she is shown to be able to speak telepathically with Nanoha. When Fate is abused once more, Arf tried to chew Precia out on this, but is then injured and sent to Earth.
Nanoha's friend Alisa Bannings later finds her, and takes her home. Arf then teams up with Nanoha, and they go to the Garden of Time. She offers to take care of Fate after she fainted about finding out that she was a clone of Precia's other daughter, but leaves when she thinks that the others might need her help. Fate almost dies the same way like her mother, and Arf fails to get her. Fortunately, Nanoha does. This drives her to tears at Nanoha's friendship towards Fate. She is arrested for the crimes that Precia committed. However, she, like Fate, might be proven to be innocent.