9. Sierra Joining The Heist.
In season three of Money Heist, the heavily pregnant Inspector Alicia Sierra proves a formidable opponent to The Professor and the gang. Most notably, she sews seeds of conflict between Raquel and The Professor, and engineers the shooting of Nairobi by luring her to the bank window with a chance to see her son, Axel.
However, thanks to revelations made by Rio in season four, Sierra is fired from the police force, and possibly up for charges herself. Fleeing from the Police, Sierra manages to track down The Professor, and season four ended with her pointing a gun at The Professor's head.
An increasingly popular fan theory proposes that, with nothing left to lose, Sierra will want in on the heist, to set up a new life for herself and her child.