A leftist political novice who has promised to be a champion of his country’s poor has been sworn in as Peru’s new president.

The humble two-story, adobe home of the Castillo family, located in one of the poorest districts of Peru deep in the Andes, feels a little empty now.

A teacher in one of the poorest communities in the Andes who has never held office is now Peru’s president-elect after officials in the South American country declared hi...