"Activating" the dummy actors?

There is a "Change character name" event, but when I add one during the Locke intro scene - "Change character $31's name to $31 (Ultros)" - it changes LOCKE's name for some reason, even though he is referred to as character $01 above.

yes it is here:

Code:

CC/A6CA: 7F    Change character $01's name to $01 (LOCKE )

What you want to do cannot work. Characters and actors are not the same thing. Both usually match for the main cast but character refer to the 14 games characters and 2 guests (0E and 0F) while actors are the 64 entities editable in FF3usME. Characters $31 doesn't exists and cannot exists because he would require a memory block for stats names and learned magic and there is only room for 14 (simply put, in fact entity $31 is used in event but it's an generic active party member, it does not refer to someone in particular).

In details, if your first parameter of command $7F is below $31, it will multiply the parameter by 37 (size of a character memory block). Anything above 7F 0D XX is out of bound in memory for this purpose (storing a name in RAM). If your parameter is $31 or above, the game does other stuff because $31-$34 are party members I think and camera is $35 (?). Command $7F details are at $C0A03A, see also $C09DAD that is the first thing called and does the calculation of memory offset to store the character name.