The game should preserve customization settings between play sessions, via Flash's
Local shared object storage feature. If you see a brunette girl when you close the game, then you ought to also see a brunette girl when you next launch the game.
If that's
not happening then there may be something unusual about the way that you're using the game. For example: if you're running it in an Incognito browser window then it's conceivable that your customization changes would be totally discarded ... but more importantly
you shouldn't be running the game in a browser window at all. Use the standalone
Flash Player Projector instead.
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Assuming that the session-persistence stuff isn't sufficient, you'll need to delve into more technical operations.
If you're using the
SDT Loader, then you can save your preferred charcode into a file, name it
code.txt, and then place it in the
Mods\$INIT$ folder (and the
Mods\$RESET$ folder if you want to be thorough). For stuff
beyond the standard charcode (such as SWF dynamic hairstyles and SWF costume mods) you can include those SWF files in the relevant folders ($INIT$, $RESET$) and then list them in the local
mods.txt files. But it may be easier to register those SWF files with sby's
moreclothing mod and then fully customize the character in-game (via the selector buttons, RGB sliders, etc), then generate a charcode, and finally dump this charcode into the
code.txt file (as described above).
If you're using the
vanilla game then you'll need to figure out how to edit SOL files. Find ChoIrrumatio.sol (note: there may be several copies on your local disk if you've run various copies of SDT over the years) and open it in Minerva. Flip the standardChar option to True, and set the standardCharID to the appropriate number. Nope nevermind this won't actually work.
... then you'll need to find advice from someone more clever than me. Or use the Loader instead.