I actually just coded a scene using this! What I did was remove the display name from a background character and position the speechbubble from the background character to the unknown character. To make it seem like the “unknown character” is talking, just type in
&CHARACTER starts talk_armscrossed_neutral_loop (or whatever animation u need)
then have the background character without a display name talk and position the speechbubble to the character u desire 
hope this helped, and let me know if it doesnt make sense lol