The main character of this poem is the speaker himself, who could very well be the poet, William Wordsworth. The speaker is enthusiastic about nature and all nature has to offer and teach people; he even claims that nature is the best teacher one could have, far better than books. Books only seem to cause "toil and trouble," whereas nature blesses us with health and truth, beauty and wisdom. The speaker believes that the songs of the birds in the forest contain more wisdom than any book, penned by any "sage," could ever do. He draws a distinction between the illuminating wisdom of nature, which casts a light on everything, and the literal darkness of reading inside and failing to see the sun's rays on the landscape. When one only reads about life, rather than going outdoors to experience it, one's mind remains in this figurative darkness.
The "Friend" addressed by the poem's speaker may, in fact, be the reader herself. The speaker tells this friend to get "Up! up!" in the first line of the poem, to put down her book and go outside to let nature be her teacher. The possibility that the speaker is Wordsworth and that the friend is the reader is a strong one as a result of the poem's title, "The Tables Turned." The tables are not necessarily turned if the speaker simply tells his friend to stop reading for a while and go outside, but the tables are turned if the speaker is Wordsworth and the friend is the reader. Here, then, we would have a writer of a book telling the book's reader to put down that book, even as the reader reads his work, and go outside; moreover, he tells the reader that it would be far more educational for us to go outside than to read his book. A writer telling a reader to put down the book and go outside certainly effects a turning of the tables.