underline vs emphasize

underline is a noun but emphasize is not a noun.

Word	Noun	Adjective	Verb	Adverb
underline	Yes	No	Yes	No
emphasize	No	No	Yes	No

As verbs, underline and emphasize are synonyms defined as:

Other synonyms of underline include emphasise, underscore.

As verbs, emphasize is a hypernym of underline; that is, emphasize is a word with a broader meaning than underline:

Other hypernyms of underline include accent, accentuate, emphasise, punctuate, stress.

underline (noun)	emphasize (noun)
a line drawn underneath (especially under written matter)

underline (verb)	emphasize (verb)
give extra weight to (a communication)	to stress, single out as important
draw a line or lines underneath to call attention to	give extra weight to (a communication)