The speaker of "Tintern Abbey" is the poet, William Wordsworth, himself. This isn't always the case. Think about all the poems that use the first person, but for which the "I" doesn't necessarily represent the historical poet (John Donne, we're looking at you).
There are a few clues in "Tintern Abbey" that tell us that the speaker is Wordsworth himself. First of all, the full title of the poem tells us exactly where, when, and under what circumstances the poem was written: "Lines Composed a Few Miles above Tintern Abbey, on Revisiting the Banks of the Wye during a Tour, July 13, 1798." So we know that the poet himself visited the spot along the "sylvan Wye" (56), and that he'd been there at least once before (he say's he "revisiting" it). Plus, Wordsworth actually had a sister (Dorothy) who lived with him for a lot of his adult life and who did travel with him on his second trip to the Tintern area.
OK, so we're convinced that the speaker of "Tintern Abbey" is, in fact, the poet himself. So then why do we keep calling him "the speaker" instead of calling him by his name? Because we have to keep the speaker straight from the boyish past self that he describes in the poem. One of the major themes of "Tintern Abbey" is the way that people change over time, so the speaker frequently refers to his past self. This past self is fundamentally different from the person the speaker has become. To differentiate the two, we refer to the speaker of the poem – the present-day Wordsworth – as "the speaker," and his boyish past self as "William." He doesn't really imagine a future self. In the final stanza, he imagines that he's already dead. Dorothy, his sister and the third "character" of the poem, could perhaps represent his future self.
So what kind of person is the speaker? How is he different from the boyish "William"? The speaker claims to be more mature. He's learned how to observe a divine "presence" (94) in nature that the boyish "William" couldn't see because of his "thoughtless youth" (90). Hmm…if he weren't talking about himself (or at least his past self), we might call the speaker kind of arrogant.
We'll withhold judgment, though. Let's see how he interacts with his sister. Wait, he hardly interacts with her at all! We don't even know that she's with him until he addresses her in line 114. That's just about three quarters of the way through the poem! OK, maybe he was too wrapped up in his own impressions and recollections to talk to Dorothy until then. After all, sometimes we can be with people we know really well for hours without exchanging a word.
The speaker tells Dorothy that she reminds him of the way he used to be. This would be kind of sweet, except that he has already described his past self, the boyish "William," as "thoughtless." The speaker suggests that Dorothy will "mature" (138) the same way that he did himself, and he looks forward to seeing it happen. This seems a bit condescending. After all, who wants to hear their big brother say, "Aww, you're just like I was five years ago. I've matured so much since then. But don't worry, you'll mature, too!"
So the speaker's character could be construed as a bit arrogant or condescending, because he seems to believe that he's reached the apex of understanding nature, something that's not attained by either his past self, the boyish "William," or his sister Dorothy. But what's he like as a speaker? Does his way of expressing himself cause us to sympathize with him despite his possible arrogance?
The language the speaker uses is very conversational. He does sound as though he's chatting to his sister using "the real language of men" (see the "In a Nutshell" section for more on that). But in places where he gets more excited or emotional, the sentence structure gets correspondingly more wonky. In the moment that he considers the possibility that his whole theory about a divine "presence" in nature is just "vain belief" (50), he breaks off, interrupting himself twice with dashes. He gets so emotional that he calls out to the river Wye as though it's a real person (this is called "apostrophe" – check out "Symbols, Imagery, Wordplay" for more on that particular poetic device):
Be but a vain belief, yet, oh! how oft –
In darkness and amid the many shapes
Of joyless daylight; when the fretful stir
Unprofitable, and the fever of the world,
Have hung upon the beatings of my heart –
How oft, in spirit, have I turned to thee,
O sylvan Wye! thou wanderer thro' the woods,
How often has my spirit turned to thee!
Even though the speaker can be a bit condescending, he still has the occasional doubt. And that doubt keeps him human, and can help to keep us from being put off by his occasional arrogance as we read.