"There was no person when I looked at him," the 21-year-old singer recalled during the second installment of an interview airing on ABC's "Good Morning America." "He had no soul in his eyes. Just blank."
Brown was arrested Feb. 8, hours after he was accused of beating Rihanna after the two argued over another woman. He later pleaded guilty to felony assault.
In the interview, Rihanna, who is no longer with Brown, described him as "definitely my first big love." She said their relationship was intense.
"The more in love we became, the more dangerous we became for each other -- equally as dangerous," she said. "It was a bit of an obsession almost."
The attack occurred in Los Angeles' Hancock Park neighborhood as Brown drove a rented sports car. She said it was triggered when she saw a text message on his phone from another woman and confronted him about it. She said he lied about it and "I wouldn't drop it."
A search warrant affidavit filed in the case stated that Brown hit, choked and bit Rihanna and tried at one point to push her from the car.
"It was ugly," she said.
During the beating, the singer said she had no idea how it would end: "That's all I kept thinking the whole time, 'When is it going to stop? When is it going to stop?"'
Brown was sentenced to five years' probation, six months of community labor and a year of domestic violence counseling after he pleaded guilty to felony assault.
In portions of the interview aired Thursday, Rihanna warned other women facing domestic violence to not let themselves become blinded by love. She also said she regretted going back to Brown, saying it sent the wrong message to her fans.
Brown, 20, has apologized to fans and has said he has repeatedly apologized to Rihanna for the attack. Brown will recount his perspective in an interview to air Friday on MTV.
Despite the beating, Rihanna said she still cares about Brown.
"I don't hate him at all," she said. "I actually love and care about him and I'm concerned about him doing well. I want him to do well -- have a great career, have a great life, and grow up. Just take this as something that you had to go through to grow up and learn."
Rihanna's interview coincides with the debut of her new single, "Russian Roulette," from her upcoming album, "Rated R." More of the ABC interview will be included in Friday evening's news magazine "20/20."