Not sure if I understand it correctly, but your main character whenever you load a map in sandbox will always be Maya, its the default. Now, you can change the appearance by pressing "O" and save it by clicking to the Save as Preset button. However you can change your default Maya character to always start with the preset you chose. I think this works with other characters too after all.. There is a little tick icon at t the top left of each preset, ify you click on it it turns to green and from there when you load the game, your character will "wear" that preset:
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