unus

Unus, una, unum declines like a first and second declension adjective everywhere except two slots: the genitive singular is unius and the dative uni for all three genders, the same pattern as totus, solus, nullus and alius. It carries a second sense of only or alone, and unus ... alius sets one against another. Its plural looks strange but is real, used with nouns that exist only in the plural, so una castra is one camp.