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It's not like he wants to share his secrets with her or teach her anything. She isn't an "impressive" duelist either. There must be a reason he kept her with him.

I assume Nihilus used her due to her ability to locate worlds rich in the Force. Perhaps due to the fact that Nihilus' ability to mentally function properly became a problem and it was harded for him to focus to use the Force?

He had a nihilistic need to have her see the world as he does.

That doesn't count. Unless he had some weird Wound fetish.