notus

Notus, nota, notum is the perfect participle of nosco doing duty as a full adjective: "having been got to know", and so known or familiar. The person to whom something is known goes in the dative - nomen mihi notum. Comparative notior, superlative notissimus, both regular.

Its negative ignotus is doubly useful, since it covers both "unknown" and, of a person, "obscure, of no family". Keep notus apart from two look-alikes: nota, the feminine noun meaning a mark or a note, and Notus the south wind, a Greek loan that only happens to be spelled the same. English notable, notice and notorious all descend from the family, notorious having picked up its bad smell somewhere along the way.