After being orphaned as a child, Prince Myshkin is adopted by a kind benefactor who sends him to Switzerland to cure his epilepsy. He is called an "idiot" because of this disease, which sometimes debilitates him, and because his compassion leads him to take actions that are incomprehensible and seemingly idiotic to others. In his late 20s when the novel begins, his love and compassion draw people to him, while at the same time they often ridicule him. In the end his attempt to save a lost woman—Nastasya Filippovna—ends up destroying her as well as him and his rival Rogozhin.
Parfyon Semyonovich Rogozhin, in his late 20s, is driven by a dark sensuality, and after falling in love with Nastasya Filippovna, he will stop at nothing to have her, even though he knows she hates him. He brings her 100,000 rubles, which she throws in the fire, and then spends the rest of the novel trying to marry her, finally killing her when she accepts him in deepest despair.
The beautiful and deep-souled Nastasya is violated at an early age by her lustful guardian Totsky; she lacerates herself for giving in to him. Feeling dirty, especially after Totsky tries to "sell" her to Ganya on her 25th birthday, she runs off with Rogozhin, even though she loves Prince Myshkin. She spends the novel running between these two men, neither of whom she can accept for different reasons, and ends up being knifed by Rogozhin.
Aglaya Ivanovna Epanchin is the spoiled and eccentric youngest daughter (age 20) of the Epanchins. She falls in love with Prince Myshkin but cannot get over her jealousy of Nastasya Filippovna, whom the prince loves only out of pity. She cannot accept the prince's inhuman standard of compassion, which demands that he choose Nastasya over herself after she forces his hand. On the rebound Aglaya marries a fake Polish count over the objections of her family and ends up converting to Catholicism and becoming estranged from her family.
Lebedev, about 40, sucks up to the rich and the powerful and also inserts himself into everybody's business. He has a large family and has recently become a widower. To supplement his income he becomes a moneylender. He plays several nasty tricks on Prince Myshkin, who easily forgives him.
Ippolit is a nihilist who has not completely given up the idea of an afterlife. At one point he reads a long manifesto to a circle of acquaintances at Prince Myshkin's impromptu birthday party, in which he expresses his anguish about dying young and attempts to appear heroic. He is spiteful and bitter because he feels as if nobody loves him.
Ganya, in his late 20s, lives with his parents and sister and brother in a large apartment in which the family takes in boarders. The family also depends on Ganya's income. After he agrees to marry Nastasya and she turns him down, Ganya quits his job as General Epanchin's secretary and lives off his brother-in-law Ptitsyn. He then tries to ingratiate himself with Aglaya, another rich woman, but he has never loved either of them.