How to get a text character based on a number position in the alphabet in a QGIS expression

You can use this QGIS expression: replace the number 1 at the second last line with any number from 1 to 26 to get the corresponding letter - if you set it to 5, it will return E:

map_get(
    hstore_to_map( 
        array_to_string (
            array_foreach (
                generate_series (1,26),
                to_string (
                    @element || '=>' || char (@element+64)
                )
            )
        )
    ),1
)

You could use a custom variable instead of the number to be more flexible and get the current value of the variable from somewhere else.