I still haven't found her myself from what i can tell i manage to find a hidden hint here in itch.io and in night fox work twitter page there a screenshot showing lexy the shark in the bar area at 1 o'clock that is the "hint" on how to unlock her there are 3 nights that you work near the bar night 3, night 4 and night 5 so far, but there must be also a condition in order to unlock lexy the shark however what the condition is still a mystery.
Anyway the key hint on finding lexy is she can be found in the bar area at 1 o'clock and as i mention before there are 3 nights where she can be found.
Edit: Here there twitter page about lexy the shark https://twitter.com/NightFoxWorks/status/1710817161105584129