The brightness setting should be stored in the file /sys/class/backlight/acpi_video0/brightness or /sys/class/backlight/intel_backlight/brightness
Changing the number in one of these files should change the brightness.
To set the default brightness, add an entry to /etc/rc.local, so that the bottom of the file looks like:
echo [x] > /sys/class/backlight/acpi_video0/brightness
depending on which file is on your system.
echo [x] > /sys/class/backlight/intel_backlight/brightness
[x] is a number usually from 0 to 10, but it varies between systems.