I created a unique name for england in descr_names and text\names and used that:
Code:
monitor_event FactionTurnStart FactionIsLocal
spawn_army
faction slave, sub_faction england
character Gigantus, named character, age 38, x 207, y 178
unit Billmen exp 1 armour 0 weapon_lvl 0
end
terminate_monitor
end_monitor
Didn't display - not even an empty color bar where the name would be. Made an overlay from first selecting the faction character and then the slave character.

Gave the name to the French leader - no display.
Used the following (descr_strat) slave faction entry and got the name displayed (see pic below):
Code:
character sub_faction england, Gigantus, general, male, age 26, x 209, y 178
army
unit Spear Militia exp 0 armour 0 weapon_lvl 0
unit Spear Militia exp 1 armour 0 weapon_lvl 0
unit Crossbowmen exp 0 armour 0 weapon_lvl 0
unit Crossbowmen exp 1 armour 0 weapon_lvl 0
Changed the sub faction entry to france and again no name display.

Added the name to the slave faction and still no display for the captain, named character displayed correctly - used 'display names' for pic.

Summary of slave name use (else no display):
script - from slave faction's list
descr-strat - from sub faction's list