*We learnt about her posthumously in a mobisode. I seriously don't think we are ever going to meet her. Ever.
*Whoever she is, she's most likely not someone on the plane, or if she was then she would have to be in the tail section since Arzt died before meeting the tailies and many of the tailies were captured before meeting the mid-sectioners.
*She could be Libby. We do not know her reason for boarding Flight 815 and Arzt sat in the middle section of the plane, whilst Libby was in the tail section.
*If we're assuming she's someone we've already met, she could be
**Cindy - A native Australian; she and Arzt could have known, or not have known, that they were on the same plane.
***Cindy was in a relationship with Gary Troup, so unless she had recently broken up with him before the flight, it seems unlikely.
****That relationship is implied in his book, but not confirmed to be true by either the producers or the fact that it's a work of meta-fiction. If she was in a relationship with Troup, she could have been seeking another, outside relationship.
**Lindsey - Claire's aunt; little is known about her at the moment, including her marital status.
*She might not be anybody we've met or will meet, it could just be an explanation of why Arzt was on the plane and to provide background to his character.
**That is a very probable explanation. It's also possible it was someone we ''do'' know or were ''supposed'' to know, but it may never have come to fruition.