When you create a general character in the start_pos tables in TWeak (or through any other means), the game automatically assigns them a random appearance and general unit card when the game starts up for the first time. And, each time you change the start_pos, the non-stock generals (in other words, ones you made) will change appearance and card. If you start a new game with the same start_pos, they stay the same as the last time you started a game with them. So it has to be something in the start_pos that is randomly generated somewhere when it's created.

Is there any way to tell the game to use specific unit parts on the models and unit cards for the characters? The Daimyo always has special unit_variant files for battle and the campaign map, not to mention his own card. I think stock generals always stay the same even when the start_pos is edited (though I'm pretty sure they're not forced to look a certain way.)