alpha vs alphabetic character

alpha and alphabetic character both are nouns.

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

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

alpha: the 1st letter of the Greek alphabet
alphabetic character: the conventional characters of the alphabet used to represent speech

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

alpha (noun)	alphabetic character (noun)
the 1st letter of the Greek alphabet	the conventional characters of the alphabet used to represent speech
the beginning of a series or sequence

alpha (adjective)	alphabetic character (adjective)
early testing stage of a software or hardware product
first in order of importance