At the end of the last book Rosie’s primary conscious personality split off and followed her memory of her little sister while the secondary or unconscious personality stayed in her body. As a result, her secondary personality was able to develop independent of her.
O’Brien developed the “second Rosie” much as any writer would develop a new character, though there were similarities between her and “first Rosie”. By the time I got about halfway through, I was reading two completely separate characters, but the growth was so subtle and so carefully done.
An interesting and exciting read. I can’t wait for the next book in the series.