The feminine form of Angels probably begin with Hellenization and mixing the Hebrew and Greek cultures, particularly later within the Roman Empire when Christianity began to emerge and Gnostic Christianity was a powerful element. However, Gnostic Christianity fell into decline and was eventually persecuted by other sects of Christianity as well as, starting in 180AD with Irenaeus the early Christian Bishop, who wrote the book titled, "On the Detection and Overthrow of the So-Called Gnosis" which was later translated to Latin and called "Against Heresies". This remains the major source of knowledge about early Gnostic Christianity. Some take his writings to a more factually based description of Gnostic Christianity while others take it to be a more political description of Christianity, as one would expect of one describing an adversary, for example like Hitler's descriptions of the Jews, perhaps unfairly.
Apart of the early Christian Bishop Irenaeus, the primary source of knowledge is the Nag Hammadi Library of Gnostic Scriptures, which were hidden in the Egyptian desert and just discovered in the twentieth century. There were thirteen codices containing a complete collection of books considered sacred or worthy of saving by the gnostics. Many believe the books were buried to save them from burning during the periods of persecution. The most published work from this "Bible" (Greek for collection of books) is the Gospel of Thomas. However, there were many other books in collection, including Plato's Republic. What is known is that the Gnostics were preserving Greek Knowledge. The word "gnosis" is Greek for knowledge.
While other Christian sects were busy eradicating Greek history, Greek lore, and the Greek language, the Gnostics appear to have been preserving this "gnosis" hence their label "Gnostics". One word of warning: consensus driven wikipedia and other "scholarly" sources ususually talk of gnostics as a form of mysticism, a more "magical thinking" version of Christianity, and they argue that their "magic thinking" for them was "knowledge", however, I strongly disagree, based on my knowledge and in-depth research and analysis, the opposite was true.
I believe the Gnostics kept alive the Greek Goddesses within Christianity, for example the Greek Goddess Nike which became conflated with the Roman Goddess Victory and with "Guardian Angels". There are no descriptions of guardian angels in the Bible. They were male messangers and special mighty warrior messangers for the LORD. But the Greek Goddess Nike flew overhead providing protection. And the Roman Goddess Victoria did so especially for the Roman Legions, providing them with Victory. It seems to me that at an early date in Christian history, the Gnostics began conflating Greek Goddesses with Hebrew Angles, as they carried on the lores and traditions and languages of the Greek, as well as the Hebrew. The other Christians, however, were busy eradicating the language, lore and history of the Greeks where the Greek language predominated. Only the Byzantine preserved the language.
Later on, art work of angles and goddesses would be painted or sculpted. They would be labeled as both. This is part of the process of conflation.