"je ne travaille pour personne". The rule is 2 negatives words per negative sentence. "ne" is one and then "pas" or "jamais" or "que" is usually the other one. But it can be replace by a negative word instead. Like in English, "someone" becomes "no one", in French we have the same logic. "quelqu'un" becomes "personne". Other negative words like that are "rien" (nothing) and "nulle part" (nowhere) and "aucun" (none).

Je ne travaille pas pour personne.

Je ne travaille pas pour quelqu'un ( personne ) .