Roman vs Italian

Roman and Italian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Roman	Yes	Yes	No	No
Italian	Yes	Yes	No	No

As nouns, Italian is a hypernym of Roman; that is, Italian is a word with a broader meaning than Roman:

Roman (noun)	Italian (noun)
a resident of modern Rome	the Romance language spoken in Italy
an inhabitant of the ancient Roman Empire	a native or inhabitant of Italy

Roman (adjective)	Italian (adjective)
of or relating to or supporting Romanism	of or pertaining to or characteristic of Italy or its people or culture or language
of or relating to or derived from Rome (especially ancient Rome)
characteristic of the modern type that most directly represents the type used in ancient Roman inscriptions