Both
Xanthe and Xanthi are perfectly legitimate transcriptions from Greek. In fact, the name of the city in Greece is transcribed as Xanthi. I and E are used interchangably to represent the letter eta. That is why you can see
Eleni and
Elene,
Kyriaki and
Kyriake, and Zoi and
Zoe, etc. In either case I rather prefer the Greek pronounciation, KSAHN-thee.