u vs alphabetic character

u and alphabetic character both are nouns.

Word	Noun	Adjective	Verb	Adverb
u	Yes	Yes	No	No
alphabetic character	Yes	No	No	No

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

u: the 21st letter of the Roman alphabet
alphabetic character: the conventional characters of the alphabet used to represent speech

Other hypernyms of u include letter, letter of the alphabet.

u (noun)	alphabetic character (noun)
the 21st letter of the Roman alphabet	the conventional characters of the alphabet used to represent speech

u (adjective)	alphabetic character (adjective)
(chiefly British) of or appropriate to the upper classes especially in language use