roundhead vs someone

roundhead and someone both are nouns.

Word	Noun	Adjective	Verb	Adverb
roundhead	Yes	No	No	No
someone	Yes	No	No	No

As nouns, someone is a hypernym of roundhead; that is, someone is a word with a broader meaning than roundhead:

roundhead: a brachycephalic person
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of roundhead include individual, mortal, person, somebody, soul.

roundhead (noun)	someone (noun)
a brachycephalic person	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential