FireRed's {KUN} placeholder always expanded to nothing: the frlg dialect has no placeholder source, so the text IR dropped it. On the cart it is gExpandedPlaceholder_Kun or _Chan by the player's gender (pokefirered/src/string_util.c:386), empty on the US cart and くん/ちゃん on the Japanese one.
The frlg dialect now names those two labels, and {KUN} prints the script cache's string for the player's gender, so a translation's text override for either label reaches every line that uses the placeholder. With the cache not loaded, or both strings empty as on the US cart, it still prints nothing and never asks for the gender. A label whose own text holds {KUN} expands it to nothing rather than recursing. Emerald keeps reading {KUN} from its placeholders.
thibautbus