I vs letter of the alphabet

I and letter of the alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
I	Yes	No	No	No
letter of the alphabet	Yes	No	No	No

As nouns, letter of the alphabet is a hypernym of I; that is, letter of the alphabet is a word with a broader meaning than I:

I: the 9th letter of the Roman alphabet
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of I include alphabetic character, letter.

I (noun)	letter of the alphabet (noun)
the 9th letter of the Roman alphabet	the conventional characters of the alphabet used to represent speech
the smallest whole number or a numeral representing this number
a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)