Vee negative voltage output LCD
in the datasheet of this LCD it says that pin 15 is Vee and is a negative voltage output.


On this particular LCD module, pin 15 (Vee) is indeed a negative output voltage, which is needed in order to produce the necessary voltage on pin 3 (Vo). This is shown on datasheet page 6 - here is an extract:



and again on datasheet page 10, where it shows that you should aim for 6.5V at room temperature on pin 3 (Vo) (measured with respect to Vss). Since Vdd is only 5V, you need the negative voltage from pin 15 (Vee) on the external contrast potentiometer, to achieve Vo (which they also call "VOP" and "\$\small V_\textrm{OP}\$") of 6.5V:



That is the reason for the "Note" at the bottom of that table, indicating that the circuit shown (similar to that on page 6 of the datasheet) is required to provide the correct voltage on pin 3 (Vo).

Try measuring the voltage on pin 15 (Vee), with respect to pin 1 (Vss), and you should find it is a negative voltage.

Some other LCD modules require a negative voltage on the contrast potentiometer (so that Vo can be greater than Vdd) but don't provide that negative voltage themselves. Then we need to provide one e.g. from an oscillator, capacitor and diode arrangement. This LCD module removes the need for those external components, by supplying that negative voltage for use with the contrast potentiometer.


  From my experience pin 15 is usually LED+


You could analyse some of the circuit on the back of the LCD module's PCB, to confirm that pin 15 does not go to the LED backlight.