Desirée

A strange creature found sleeping in the depths of the ocean by a prince, Captain Rei.
When awakened she was mistaken by Rei to be a succubus of some kind, but it's soon discovered she's actually a shapeshifter (the succubus nickname stuck, tho).
She woke up without a single memory, but was found with an old book who she and the captain decipher a bit more day by day, and which tells not only of her origins, but of those of the whole universe...