Yankee vs northern

Yankee is a noun but northern is not a noun.

Word	Noun	Adjective	Verb	Adverb
Yankee	Yes	Yes	No	No
northern	No	Yes	No	No

Yankee (noun)	northern (noun)
an American (especially to non-Americans)
an American who lives in New England
an American who lives in the North (especially during the American Civil War)

Yankee (adjective)	northern (adjective)
used by Southerners for an inhabitant of a northern state in the United States (especially a Union soldier)	situated in regions of the north
	in or characteristic of a region of the United States north of (approximately) the Mason-Dixon line
	coming from the north; used especially of wind
	oriented toward the north