(CNN) -- The Ohio man who imprisoned three women in his Cleveland home for a decade will speak at length during his sentencing Thursday, delivering a statement that his sister promises will allow people to see "the other side of Ariel Castro."
Castro pleaded guilty last week to 937 counts, including murder and kidnapping, in a deal that dropped a possible death penalty in exchange for life in prison plus 1,000 years.
He'll give a rather lengthy statement, explaining his life and who he really is, his sister, Marisol Alicea, told CNN on Wednesday night.
"(People will) see the other side of Ariel Castro ... not the monster that everyone thinks he is," she said, adding that she was in no way defending her brother.
"He must pay for what he did."
Alicea said she doesn't plan on attending the sentencing with others in her family, fearing the evidence will be too graphic.
The sentencing hearing is expected to last hours and include witness testimony and evidence, a Cuyahoga County court source, who spoke on condition of anonymity, told CNN.
Prosecutors want to make sure there's a record going forward in case of a future appeal, should Castro want to try to get out of prison, said the source who was not authorized to speak publicly on the matter. Prosecutors also want the court and the public to understand the impact Castro's actions had on his victims and the community.
At least one of Castro's victims, Michelle Knight, will likely make an impact statement during the hearing, Alicea said. An official with direct knowledge of the investigation also said Knight intends to speak.
In a handwritten note, posted Wednesday on the Cleveland Police Community Relations Facebook page, Knight said she was overwhelmed with the support she has received from "complete strangers."
"It is comforting. Life is tough, but I'm tougher," she wrote. "Just when the caterpillar thought the world was over, she became a butterfly. Thanks."
It is unclear if the other two women -- Gina DeJesus and Amanda Berry -- will address the court. They could make a videotaped statement, or a family member could talk on their behalf.
Prosecutors also have submitted an evaluation of Castro's confinement and abuse of Knight, DeJesus and Berry that was compiled by acclaimed psychiatrist Frank Ochberg, considered a pioneer in trauma science.
The evaluation was part of the prosecution's pre-sentencing report, which has been submitted to the court.
The evaluation -- using statements, medical records, videotaped interviews and transcripts -- painted a horrifying picture of physical and emotional abuse at the hands of Castro that included brutal beatings and repeated rapes that resulted in pregnancies that he would end by punching the women in the stomach.
"He appeared to be evolving in an ever more dangerous direction, capturing younger and younger women, telling his captives he was hunting for replacements,"Ochberg wrote.
Castro abducted Knight, Berry and DeJesus separately over a two-year period between 2002 and 2004, according to investigators.
The women and Berry's 6-year-old daughter were held in Castro's 1,400-square-foot home. DNA tests have confirmed that Castro is the child's father.
In the evaluation, Ochberg wrote that Knight -- who was kidnapped first -- suffered "the longest and most severely."
"But it was Michelle who served as doctor, nurse, midwife and pediatrician during the birth (of Berry's child). She breathed life into that infant when she wasn't breathing," he wrote.
"At other times, she interceded when Castro sought to abuse Gina, interposing herself and absorbing physical and sexual trauma. But each survivor had a will to prevail and used that will to live through the ordeal."
The women were freed in May after Berry shouted for help while Castro was away.
Neighbor Charles Ramsey said he heard their cries as he was sitting down to eat.
"I see this girl going nuts trying to get out of a house," he told CNN affiliate WEWS. "I go on the porch and she says, 'Help me get out. I've been in here a long time.' "
Finally free, Berry pleaded for a phone.
"Help me, I am Amanda Berry," she frantically told a 911 operator. "I've been kidnapped, and I've been missing for 10 years. And I'm here, I'm free now."
In early July, Berry, DeJesus and Knight released a YouTube video offering their thanks to all those who have helped them since they were freed. They have not faced their captor and tormentor since their rescue.
"I want to thank everyone who has helped me and my family through this entire ordeal. Everyone who has been there to support us has been a blessing," Berry said in the video. "I'm getting stronger each day."
CNN's Chris Boyette, Ronni Berke, Ashley Fantz and Martin Savidge contributed to this report.