The speaker appears to be a version of Shelley himself. As in other poems (such as "The Mask of Anarchy," for example), Shelley recounts a dreamlike vision in which a metaphorical picture of reality appears. The vision and the way it is presented to the reader are similar to the material in the prophetic books of the Bible, especially Revelation, the last book of the New Testament. Therefore the speaker—Shelley, that is—is a kind of prophet who is unfolding to mankind, his readers, a secret about the cosmos and about the meaning of existence.
In his vision, the speaker describes a huge throng of people rushing headlong, crowding around, or being chased by a chariot driven by a Janus-faced figure. We are made to understand that this crowd represents all of mankind. The crowd is depicted as helpless, not understanding what they are doing or why they are on this "public way, thick strewn with summer dust":
. . . a great stream
Of people there was hurrying to & fro
Numerous as gnats upon the evening gleam,
All hastening onward, yet none seemed to know
Whither he went, or whence he came, or why
He made one of the multitude, yet so
Was borne amid the crowd as through the sky
One of the million leaves of summer's bier.—
Old age & youth, manhood & infancy,
Mixed in one mighty torrent did appear,
Some flying from the thing they feared & some
Seeking the object of another's fear . . .
As stated, a chariot is being driven through this crowd, and the driver is a loathsome figure which, one would at first guess, represents Death:
So came a chariot on the silent storm
Of its own rushing splendour, and a Shape
So sate within as one whom years deform
Beneath a dusky hood & double cape
Crouching within the shadow of a tomb,
And o'er what seemed the head, a cloud like crape,
Was bent a dun & faint etherial gloom
Tempering the light; upon the chariot's beam
A Janus-visaged Shadow did assume
The guidance of that wonder-winged team.
But in a few lines, after the speaker has asked what this figure is, a voice answers that it is "Life." It is as if the throng of mankind are defeated by it, beaten down by life and the cruel suffering it brings.
The famous Swiss philosopher Rousseau (1712–1778) appears, at first not even seeming human but then taking on the shape of a man, a "grim Feature" who identifies himself to the speaker and then becomes a guide to him, describing himself sadly:
Before thy memory
"I feared, loved, hated, suffered, did, & died,
And if the spark with which Heaven lit my spirit
Earth had with purer nutriment supplied
"Corruption would not now thus much inherit
Of what was once Rousseau . . .
Shelley likely invokes Rousseau because he was, perhaps more than anyone, the chief symbol of the Enlightenment, the age which preceded Shelley's own time and the ideas of which formed the basis of the French Revolution and the progressive thought of the following epochs. Rousseau functions in "The Triumph of Life" much as Virgil does in Dante's Inferno—as a symbol of the ideas to which Shelley himself adheres, and as the poet's hero and idol.
"The Wise, the Great, the Unforgotten"
These are people "chained to" the chariot. They are famous figures of the past, men and women of the political and intellectual world who in some sense achieved greatness but nevertheless ultimately failed in a quest to redeem mankind. Among them are the philosophers Plato, Aristotle (who is not named explicitly), Voltaire, and Immanuel Kant; Alexander the Great (also not actually named), the Empress Catherine the Great of Russia, and Holy Roman Emperor Leopold II; and perhaps most important of all, Napoleon. All of these characters are grand-scale representatives of humanity—that is, the masses, the throng the speaker beholds in his apocalyptic vision.
Because Shelley left the poem unfinished, we do not know the ultimate significance of all these characters and how they will presumably facilitate the themes of "The Triumph of Life," but they are a mirror of the speaker, Shelley himself. All of them appear to represent, or to be a projection of, Shelley's own thoughts and emotions.