The primary character in this sonnet is the speaker, generally assumed to be Shakespeare himself. There is a traceable theme and story that runs through the Shakespearean sonnet cycle, which supports the idea that the writer (elsewhere identified, through the use of pun, as "Will") is William Shakespeare. This poem falls within the first 126 sonnets, all of which are dedicated to an unknown "fair youth," whose beauty is the subject of most of the poems.
That is true also of this poem. The addressed subject is not named and is not identified, in this particular poem, by gender, but context indicates that it is the same man as is addressed in the other sonnets. His gender is implied in this poem through the use of the term "master"—he masters a sort of beauty which engenders "wonder" in the breasts of those who look upon it, and the speaker feels inadequate to "praise" it with his human tongue, as if the beauty is something beyond human capacity.
There are some other characters mentioned in passing in the poem—namely, the "lovely knights" and "ladies dead" who have featured in the chronicles of older writers. These older writers are viewed together as a group, wielding an "antique pen" in praise of beauty in a way the speaker sees as a prophetic anticipation of his beloved's beauty.