Eva was born as the fraternal twin sister of Juliet Violet from a one time relationship between Harriet Violet and Aaron Goldie that Aaron can barely recall. Harriet married Adamska Violet before giving birth to the twins, so he was legally recognized as their father. She was the half-sister of Morris Goldie, Chris, Lorie Goldie, and Cain Violet.
Due to Ritsuka Fujimaru's consciousness being sent into that of Juliet's friend, Ritsuka is unable to comprehend all of the sensory data in regards to humans through that connection. Ritsuka views them as Servants contracted to them, Eva appearing as Raikou. The assigned Servants may have some connection in terms of personality, but it is possible for key traits of the person to be confused due to the process. In the case of Eva, she looks nothing like her sister, who is portrayed as Stheno, while their mother is portrayed as Stheno's twin sister Euryale. This causes a fundamental misunderstanding for Ritsuka for the majority of the incident.