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Flag 0x1203 is not a flag. For Firered, you can use flags between 200-2FF. Just beware that ingame events make use of a lot of these already, you can refer to this document to see which are unused.
To make a sprite appear the easiest way is to clear a flag.
Set their person ID as 200 (or any flag for that matter) in A-Map. When the flag is set they will be hidden when you enter the map, but visible if its not set.
So:
>Set the flag earlier on in your hack.
>Clear flag when you want the person visible.
>'showsprite' command if you want them visible straight away, in the map you're currently in.