foe vs challenger

foe and challenger both are nouns.

Word	Noun	Adjective	Verb	Adverb
foe	Yes	No	No	No
challenger	Yes	No	No	No

As nouns, challenger is a hypernym of foe; that is, challenger is a word with a broader meaning than foe:

Other hypernyms of foe include competition, competitor, contender, rival.

foe (noun)	challenger (noun)
a personal enemy	the contestant you hope to defeat
an armed adversary (especially a member of an opposing military force)