There are two ways to get Lexy here a step by step guide to do it
“First Way”
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.
“Second Way”
Step 1: Have paranormal research at 400/1000 before night 6 to unlock Magic shop.
Step 2: Buy magic Necklace
Step 3: In Night 7 choose VIP room
After you done all three steps then Lexy the shark will be unlocked.