Like the astrological figure, the moon,
it means the sub-conscious self, the inner most feelings,
It also means a secrets.
Like the moon with many faces at different times of the month, the high priestess may be in many appearances to each and every reading.
She could be a teacher, a relative, a cleaner on the street, your friend.
Yet one thing is certain, she reveals some truth to questions which you have and keeps certain truths which she feels it is not the time to answer.
She also represents "blink" or the concept of action without us knowing.
Bast or Bastet was the cat goddess and local deity of the town of Bubastis "Tell or Per-Bast in Egyptian, where her cult was centered. Bubastis was named after her. Originally she was viewed as the protector goddess of Lower Egypt, and consequently depicted as a fierce lioness. Indeed, her name means (female) devourer. As protector, she was seen as defender of the pharaoh, and consequently of the later chief male deity, Ra, who was a solar deity also, gaining her the titles Lady of Flame and Eye of Ra.
The goddess Bast was sometimes depicted holding a ceremonial sistrum in one hand and an aegis in the other – the aegis usually resembling a collar or gorget embellished with a lioness head.
Bast was a goddess of the sun throughout most of Ancient Egyptian history, but later was changed to a goddess of the moon by Greeks occupying Ancient Egypt toward the end of its civilization. In Greek mythology, Bast is also known as Aelurus. ======================
Taken from Wiki.....
Incidentally, for the book of lost souls, the character's cat is known as mystery.....
Every cat is one... so is the high priestess.