Attorney General Mike DeWine's office confirmed Castro's paternity in a news release Friday. DeWine says a sample of Castro's DNA was taken Thursday and forensic scientists worked through the night on the case.
The girl is the daughter of Amanda Berry, who authorities say was held for about a decade in Castro's house in Cleveland along with Gina Dejesus and Michelle Knight.
The three women said Castro chained them up in the basement but eventually let them live on the home's second floor. Each woman told a similar story about being abducted after accepting a ride from him.
Berry, now 27, told officers that she was forced to give birth in a plastic pool in the house so it would be easier to clean up. Berry said none of the women - or the 6-year-old - went to a doctor during their captivity.
Michelle Knight, now 32, said her five pregnancies ended after Castro starved her for at least two weeks and "repeatedly punched her in the stomach until she miscarried." She also said Castro forced her to deliver Berry's baby under threat of death if the baby died. When the newborn stopped breathing, Knight said, she revived her through mouth-to-mouth resuscitation.
During his brief arraignment Thursday, Castro tried to hide his face, tucking his chin inside his shirt collar. He appeared to close his eyes during the hearing and awkwardly signed documents while handcuffed. He did not speak or enter a plea.
In court, prosecutor Brian Murphy said Castro used the women "in whatever self-gratifying, self-serving way he saw fit."
Kathleen DeMetz, a public defender assigned to represent him at the hearing, didn't comment on his guilt or innocence or object when prosecutors recommended that bail be set at $5 million. The judge, instead, ordered Castro held on $8 million bail.
Castro was arrested Monday, when Berry broke out of his run-down house and called 911 while he was away. Police found the two other women inside. The women had vanished separately between 2002 and 2004, when they 14, 16 and 20.
Police then entered the house and found the other women, who flung themselves into the officers' arms.
Berry and former captive Gina DeJesus, 22, went home with relatives Wednesday. Knight was reported in good condition at a Cleveland hospital.
Castro's two brothers, who were arrested with him but later cleared of involvement in the case, appeared in court on unrelated charges Thursday and were released.
Some relatives of Castro have said they were shocked by the allegations against him. An uncle, Julio Castro, said it's been difficult news to absorb.
"Of course we have taken it hard," he said. "We only knew one Ariel, my sweet nephew. He was a sweet, happy person, a musician. We didn't have the slightest idea of the second person in him."
Juan Perez, who lives two doors down from Castro, said Castro was always happy and respectful. "He gained trust with the kids and with the parents. You can only do that if you're nice," Perez said.
On a recent visit to Castro's rundown home, his friend Ricky Sanchez said he heard noises "like banging on a wall" and noticed four or five locks on the outside door. While he was there, a little girl came out from the kitchen and stared at him. But she didn't say anything.
"When I was about to leave, I tried to open the door," he said. "I couldn't even, because there were so many locks in there."
Castro is being held on $8 million bond.
Associated Press writers Thomas J. Sheeran, Mike Householder, Andrew Welsh-Huggins and John Coyne in Cleveland; Brendan Farrington in Florida; and Dan Sewell in Cincinnati contributed to this report.