waifu vs fictional character

waifu and fictional character both are nouns.

Word	Noun	Adjective	Verb	Adverb
waifu	Yes	No	No	No
fictional character	Yes	No	No	No

As nouns, fictional character is a hypernym of waifu; that is, fictional character is a word with a broader meaning than waifu:

waifu: a character in an anime or manga who one has romantic feelings for
fictional character: an imaginary person represented in a work of fiction (play or film or story)

Other hypernyms of waifu include character, fictitious character.

waifu (noun)	fictional character (noun)
a character in an anime or manga who one has romantic feelings for	an imaginary person represented in a work of fiction (play or film or story)