Dostoyevsky wanted to create a character that was "entirely positive... with an absolutely beautiful nature," and a good way to make such a character plausible in 19th century St Petersburg society was to make him an "idiot". He spends his youth in Switzerland in an epileptic cloud; he never attends school and at age 26 he returns to Russia. Among St. Petersburg society, he appears to be an "idiot" due to his purity and guilelessness, but he possesses a wisdom that surpasses all of the characters in the novel, in addition to a frequently keen, incisive intellect.
The episode concerning a picture of Nastasya Filippovna exemplifies how his innocent indiscretions lead others to conclude he is an idiot. He is visiting General Yepanchin in hopes of finding work when Ganya pays the General a visit. The General has encouraged Ganya to propose (to Nastasya Filipovna) and Ganya has just returned from declaring his intentions to her. Nastasya has left Ganya with a portrait of herself, and as the two men admire it, the Prince is struck by the beauty of the woman that Rogozhin has recently told him about (The Rogue). Lev Nikolaevich is then escorted to an adjacent room where he is to meet the General's wife and three daughters.
The Prince mentions in a discussion about beauty that he finds Aglaya, one of the sisters, to be almost as beautiful as Nastasya Filipovna. The four women are very surprised that he should know Nastasya, but the Prince simply explains that he has just been viewing a photo of her with the General in the other room. Everyone is shocked at this news, and the General's wife demands to see the portrait, asking the Prince to return to the General's office and fetch it for her. For Ganya, this is an outrageous indiscretion, especially given that he is also making overtures to Aglaya.
The Prince means only to do the right thing, but is somewhat removed from reality; while he has considerable emotional intelligence he has little experience in the ways of St Petersburg society. Motivated by a wish to save all of the characters surrounding him, the Prince falls in love with Aglaya out of a desire to save her, just as he loves Nastasya out of a deep pity for her. In the end he chooses the latter, but Nastasya, despite her feelings for the Prince, runs off with Rogozhin.
Rogozhin is jealous of the Prince because he knows that Nastasya has fallen for his simple-minded, spiritual intelligence.