There is no record of who played Isis the cat as a Human.
According to various sources, Isis was portrayed by Victoria Vetri.
She was not. There are no real sources for that. There was one guy on the Internet who suggested it as a possibility
, and then some IMDb editor mistook that suggestion for a fact and entered it there as such, and other sites just uncritically repeated what IMDb said without doing any basic fact-checking first. But it's definitely not her. Vetri's face is distinctly different from the actress who played Isis; her features are softer, her eye color is different, and she has a dark speck on the white of one of her eyes that "Isis" did not have.