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Niedermann is Zalachenko's son and Salander's half-brother. He's a foil for Salander in many ways. They are diametrically opposed in size, shape, and skill, but are similar in that they both have qualities that make them stand out. For Salander, this is mostly her power-brain; for Niedermann it's the fact that he has a rare condition that keeps him from feeling any pain (if you can believe this).
In Niedermann, we can also see genuine mental illness of the sort Teleborian claims Salander has. He has constant hallucinations and absolutely no qualms about using violence and killing. In spite of all this, it's painful to read about Salander nailing his feet to the floor.
Even though we are glad Salander stops Niedermann from killing her, his nailed-down feet somehow reminds us that he is a victim of his father, too, though one who undoubtedly turned into a monster. We don't get details, but we can safely assume, especially from their interactions at the end of Fire, that Zalachenko has been abusing Niedermann for a very long time and using Niedermann's strength and psychiatric condition for evil purposes.