According to national polls, the Fuerza Popular Party's leader will trail in the race for the presidency. Castillo is expected to get 40 percent of the votes in his favor while Fujimori could receive 37 percent of the votes.
The unpopularity of the right-wing candidate is tied to the inheritance she received from her father, former president Alberto Fujimori (1990-2000), who is serving 25 years in prison for crimes against humanity.
"Her father is a corrupt man and she supports all the bad things he did," said the relative of a young journalist who died in 1991 after a bomb exploded in the editorial office of the magazine Cambio, which opposed the Fujimori administration.
Keiko, who has tried to become president on two other occasions, has been accused of corruption on several occasions. In March, the Peruvian Public Prosecutor's Office requested 30 years in prison for the candidate for alleged money laundering.
According to Peruvian authorities, Keiko is accused of organized crime, justice obstruction, fraud in administrative proceedings, and false testimony.
In recent weeks, thousands of Peruvians have taken to the streets protesting against her candidacy with signs reading, "Fujimori never again," and "That family is rotten."
Keiko's popularity has always been low. In May, an Ipsos poll revealed that 45 percent of Peruvians said they would not vote for her in the upcoming run-off elections.