Most minor characters don’t have very large parts, or even personalities. Your own Engagement Ring Princess, for example. She hasn’t honestly done anything that could give her a clear personality, so you are free to fill them in with any headcanons you feel are appropriate. The fun thing about minor characters is that they’re a kind of clean slate
But there are exceptions, like Samantha or Huntess Wizard, who have had speaking roles. You can figure out parts of their personality and backstories based off of what they say, but a lot of it is still empty. It’s really just a matter of figuring out what you know, and what you don’t, and filling in the empty spaces in character development.