Petra Coronado is Cruz Coronado's mother and a former Academy student. She created the journal that Cruz used to find the cipher pieces which are created out of black marble. She was apparently killed mysteriously in her lab by a fire.
Spoiler for the seventh book: In the seventh book, Cruz finds out his mother survived the fire because of a serum she developed that was in her blood. Cruz found his mother while following her journal's last clue.