Xaphania is the leader of the rebel Angels who allied with Lord Asriel in the war against the Authority. She first appears in the third book of the trilogy, The Amber Spyglass, at a council meeting of Lord Asriel's commanders. She appears naked like all the angels of His Dark Materials. Like all other angels, she has wings and she is luminous. She commands respect due to her high rank and age.
Her background, like almost all the angels belonging to the angelic order, is ambiguous. Sometimes, she appears as a young angel too.
The angel Balthamos is the first character to refer explicitly to Xaphania. As he explains to Will Parry, she discovered that The Authority had lied to the other angels regarding his status as the original Creator and so was banished from the Clouded Mountain. This was the start of the first struggle against the Kingdom. Xaphania is the leader of the rebel angels who intervened in human evolution to give mankind consciousness some thirty thousand years ago.
Towards the end of the trilogy, and after the defeat of forces of The Authority, it is Xaphania who explains to Will and Lyra Belacqua that all the windows between the worlds must be sealed to prevent Dust from leaking away into the abyss, and that the Æsahættr (the subtle knife) must be destroyed. She also tells them that it is possible to learn how to travel between worlds, like the angels do and like John Parry did, but it would take a life time to do.