Kreia did know her - she said so to the Exile herself. Kreia has a, ahem, talent for "misinformation", though, and I could easily believe that she would mere say that she knew Kae if she is indeed Kae herself. Note also how she refers to the third Sith Lord, who cannot be revealed too soon. That third Sith lord is clearly Kreia herself, so she obviously talks about herself in the third person. She could very easily do the same on the subject of Arren Kae, methinks.
However, there are a few clues to suggest that she is indeed Arren Kae.
1. None of the masters ever mention Kreia by name. Indeed, Atris tells us that "Kreia" is not her real name.
2. When confronting the masters, Kavar mentions (without saying her name) that he thought she had died in the Mandalorian Wars. Arren Kae supposedly died on Malachor V at the end of the Mandalorian Wars.
3. When Disciple talks about Revan, he mentions that Revan was the padawan of Arren Kae. To me this suggests that Arren Kae was Revan's first master. He also mentions, as does Kreia herself, that Revan returned to his first and final master to learn how to leave the jedi order. If Kae was his first master, then Kreia must be Kae.
There are other clues, like Atton mentioning that Kreia must have been beautiful once, or that Kreia seems to hide her presence from Atris when the Exile first meets Atris on Telos, or even that Kreia admits knowing the Handmaiden's mother and yet never uses that knowledge to manipulate the Handmaiden with, but I find the above three to be the major clues.
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That's a nice persuasive text, but not for me... If Brianna was Kreia's daughter, Brianna at least sense her when she feels force. I think Atris knows much about Arren Kae. And I must say this too Handmaiden honours her mother's face not Kreia's!!!