Here a guide on how to unlock her at night 5 follow this steps and you will unlock her.
Step 1: In night 2 when the power is out choose the “search for a vent or another way out to the room” (little note you can choose whatever to do in the vent it won’t matter).
Step 2: In night 3 after replacing the camera in the repair bay “tell zero to go back in her pod” then “restock soda wand” then “Clean up blood”
Step 3: In Night 4 do “Restock energy bars” then “vacuum booths”
After you done all three steps when you begin night 5 Lexy the shark will be unlocked.