In a narrative poem, the entity telling the story is called the narrator. The narrator is different from the author, in that the author is the real person who wrote the poem, while the narrator is a fictional entity that "lives inside" the poem. As such, author and narrator can be completely different "people".
Is there an equivalent term to refer to the character who "speaks" in a lyric poem? For example, in Shakespeare's Sonnet 18:
Shall I compare thee to a summer's day?
How do you call the "I" who wonders whether he should compare his lover to a summer's day?