Hiya (هي) in Arabic is a third-person feminine pronoun: she.
In my opinion and in this series, Hiya acts as a symbol for the lost beloved, or for the beloved we have not yet encountered. Hiya stands in contrast to Anti, or you, which is a symbol for the now-present, now-known, cherished beloved.
What, really, is Hiya? Nothing but beauty and a set of positive traits that we seek in the beloved. Whether these are lost to the past, or are not yet fully realized, we can see nothing there but beauty and goodness.