I think you need to do a good ending, you can just tell Nyx that you wish for her to not use her powers for evil, that way you get the trait and you can still have her around for more scenes

There is more info in the Guide & FAQ on the main page under Master Zhen and Nyxara:

• Complete the Spirit Fighting mini-game 3 times on different days (Click the portal on the screen to go in; failing the mini-game while the shrine is red might require extra retries)
• Go to the Shrine to meet Nyxara (You can either reveal your cock for a bj scene or not)
• Do the Spirit Fighting mini-game 2 more times and Nyx will appear once more to talk about the current world ( your choice of answer doesn’t affect the quest here)
• Do the Spirit Fighting mini-game 1 more time and Nyx will appear once more (your choice of dialog doesn’t affect outcome but it'll have different dialogs):

• Why do you want to come here so badly?
• How do I know this isn’t some kind of trick?
• Wait… You are the spirit who killed Master Zhen’s son

• After her answer you’ll be presented with the following dialog choices: