I do know about that. Maybe I wrote it wrong or unclear. It should've been like this: How to change slave's personality (like Confident, Shy, Kind, etc.) with in-game methods or there is no way to do so?
P.S. According to Draupnir7, you can't.
No, you can't with in-game tools, not even with cheats. However, what you can do is to manage the way the slave speaks to you. "loving" "slutty" "professional" or "wholesome" pretty much match the personalities you want to change.
Outside the game you can go in ../data/saves and edit a save file (they are .xml, there is nothing ground-breaking there - use Notepad++ and you'll be fine). Search for slave name, and about 20 lines under it should be an entry
Code:
<personality>
<trait>NAME_OF_THE_PERSONALITY_WRITTEN_IN_CAPS</trait>
</personality>
change that trait to your hearth desire, save, and load that file in-game. Of course, you can only use existing game traits, you can't change a slave personality to be "IRONIC" or "HILARIOUS" because there are no such traits.