Nilson is a scientist working for the criminal leader Madame Marciano. He engineered the Twelve Sisters, a group of android assassins conceived as Marciano's robotic "daughters," and personally designed their distinctive gothic lolita clothing, a stylistic choice that fuels fandom discussion about his design sensibilities.

He demonstrates a deep understanding of Marciano's motivations and inner state, privately contemplating whether she feels regret over her part in the death of the legendary pirate king, Bruce. Functioning as Marciano's advisor and confidant, Nilson consistently affirms her choices. During her rare moments of doubt about her path of corruption and violence, he reassures her, reinforcing her determination to persist. His support maintains her focus on her objectives.

The character's name has appeared with spelling variations including Nielsen, Nilson, and Nielson, lacking a single confirmed standardized romanization. Details concerning his background before joining Marciano, or significant personal development or changes in his role across the series, OVAs, spin-offs, or movies, are absent. His depiction remains constant as Marciano's scientific collaborator and unwavering supporter throughout the narrative.