What part of speech is the letter I?

"I" is a pronoun. More specifically, it is the first-person singular subject pronoun.

The Word 'I' is a pronoun, the first person, singular, subjective, personal pronoun.

A personal pronoun takes the place of a noun for a specific person or thing.

A singular pronoun takes the place of a noun for one person or thing.

The first person pronouns (I and me) takes the place of the noun (name) of the speaker. The person speaking does not use their own name when talking about them self.

A subjective pronouns takes the place of a noun as the subject of a sentence or a clause.

The corresponding first person, singular, personal pronoun that takes the place of a noun as the object of a verb or a preposition is 'me'.

Example:

When I saw the posting for this job, I knew it was right for me.