U vs alphabetic character

U and alphabetic character both are nouns.

Word	Noun	Adjective	Verb	Adverb
U	Yes	No	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
a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine