Gary Seven
Who's that cat?

Isis, the cat was "portrayed" by three different black cats. Actress Barbara Babcock provided the meows and various other cat noises. Babcock's other Star Trek voice work includes that of Trelane's mother in The Squire of Gothos and Commander Loskene in The Tholian Web. In addition to her voice work, the actress appeared onscreen as Mea 3 in A Taste of Armageddon and again as Philana in the third season episode Plato's Stepchildren.

The Mego Madhouse
Fun Stuff
Roberta Linclon
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only briefly onscreen and was uncredited for her small role. Her identity remains largely unknown among many Star Trek references, listing her merely as "unknown bit-actress" or the like. Scott Dutton's website, The Complete Reference to Assignment: Earth identifies the unkown as actress Victoria Vetri. Vetri appeared in more than a handful of television series and bee films of the 60's and 70's. But the actress is perhaps better remembered by her stage name, Angela Dorian, as Playboy's Miss September 1967 and 1968 Playmate of the year. Whether the anonymous Isis actress is Vetri for certain remains unconfirmed. See the comparison below and decide for yourself!

Isis, the woman appeared

Although no reference is cited, the Star Trek wiki*, Memory Alpha states quite plainly and without question that Victoria Vetri is indeed the actress seen as Isis. See Victoria Vetri at Memory Alpha.
*What on Earth is a wiki?
Is this the face of Isis?