nemo

nemo is ne plus homo, "not a man", and it inflects on that second half: nemo, accusative neminem, dative nemini. The genitive and ablative are borrowed from nullus, giving nullius and nullo, because neminis and nemine are avoided in classical prose. It is masculine or feminine according to who is meant, never neuter: the neuter partner is nihil, "nothing".

Two constructions come with it. A relative clause after nemo est qui goes into the SUBJUNCTIVE, because it describes a type of person rather than picking out an individual: nemo est qui credat. And, as always in Latin, two negatives cancel rather than reinforce, so non nemo means "somebody", nemo non means "everybody", and neither is a mistake.