Elder Zosima is the old monk whom Alyosha chooses as his master, the third and last of a line of elders in that monastery. The narrator defines him by what he does: the elder is one who takes another's soul and will into his own soul and will.
He is about sixty-five, was a landowner and officer in the Caucasus, hears the confession of everyone and guesses at first sight what each one carries. He is cheerful, prefers the greatest sinner, has few enemies, who are silent, and many devotees, who expect miracles after his death. In the cell, before Fyodor, who presents himself as a buffoon, he answers that he should not be so ashamed of himself, and he kneels before Mitya, without explaining why.
In Book VI, the novel tells the life and teaching of Zosima, as Alyosha wrote them down from memory: his brother Markel, who asked forgiveness of the servants and the birds; the duel in which he threw his pistol into the bushes; the visitor who confessed a crime. He teaches that each one is guilty before all, that humble love should be preferred to force, that no one is anyone's judge, and that hell is the suffering of no longer being able to love.
He dies kneeling, kissing the earth, as he had taught. The next day the body smells bad, and the monastery divides. That smell is Alyosha's first test. Before dying, the elder leaves him the order to leave the monastery and live in the world.