Who was Mr.GUEST?

Mr. Guest was Mr. Utterson's head clerk.

Presently after, he sat on one side of his own hearth, with Mr. Guest, his head clerk, upon the other, and midway between, at a nicely calculated distance from the fire, a bottle of a particular old wine that had long dwelt unsunned in the foundations of his house. The fog still slept on the wing above the drowned city, where the lamps glimmered like carbuncles; and through the muffle and smother of these fallen clouds, the procession of the town's life was still rolling in through the great arteries with a sound as of a mighty wind.