nullus

Nullus, nulla, nullum is ne plus ullus, "not any", and it declines like a first and second declension adjective everywhere except two slots: the genitive singular is nullius and the dative singular nulli, both the same for all three genders. That is the pattern shared by unus, totus, solus, alius, alter, uter and neuter, the group usually taught as the UNUS NAUTA words. Learn the nine together and you never have to think about them again.

Latin divides the labour between nullus and nemo. Nullus is the adjective, going with a noun; nemo is the noun, "nobody", standing on its own. But nemo is defective, so it borrows: the standard genitive of nemo is nullius and the standard ablative nullo. Neither word is a doublet of the other.

Latin does not stack negatives the way English dialects do. Two negatives in the same clause cancel, so non nullus means "some" and non nemo "somebody", which is a deliberate understatement rather than a mistake. It gives null, nullify, annul and, through French, none of the good words but a great many legal ones.