Story highlights Ariel Castro's sister says he must pay for what he did

Castro will deliver a statement explaining his life, the sister says

He'll be sentenced Thursday; the plea deal calls for life plus at least 1,000 years

One victim, Michelle Knight, will likely speak at hearing

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.

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Photos: Photos: Cleveland's missing women Photos: Photos: Cleveland's missing women Gruesome discovery in Cleveland – On July 21, Cleveland authorities announced they had discovered the bodies of three women, later identified as Angela Deskins, from left, Shirellda Terry and Shetisha Sheeley. All three women had been reported missing. Hide Caption 1 of 11 Photos: Photos: Cleveland's missing women Suspect arrested in deaths – Michael Madison, 35, a registered sex offender, was charged in the deaths of the three women. He was arrested after investigators noticed a foul odor coming from a garage he leased and found a body inside. Police believe he killed the women over a 10-day period. Hide Caption 2 of 11 Photos: Photos: Cleveland's missing women Cleveland's missing women – Posters for missing women and girls can been seen all over metropolitan Cleveland's impoverished neighborhoods. The Cleveland police website lists 54 women as missing within its city limits, but a community leader said he thinks that number is actually in the hundreds. Hide Caption 3 of 11 Photos: Photos: Cleveland's missing women Still missing: Christina Kleckner – Christina Kleckner, 24, was last seen in Cleveland in October 2011 after an argument with her parents, according to CNN affiliate WEWS. Her father reported her missing the next day. Kleckner, a recovering drug addict with mental disabilities, was being treated for bipolar disorder and had been involved in prostitution. Hide Caption 4 of 11 Photos: Photos: Cleveland's missing women Still missing: Minerva Tripp – Cleveland resident Minerva Tripp vanished last year at age 41. Hide Caption 5 of 11 Photos: Photos: Cleveland's missing women Still missing: Ashley Summers – A photo of Ashley Summers, who was last seen in the Cleveland area on July 9, 2007, beside an age-progressed rendering of her. Hide Caption 6 of 11 Photos: Photos: Cleveland's missing women Still missing: Christina Adkins – Christina Adkins was last seen in Cleveland in January 1995. She was 18 years old and five months pregnant when she disappeared. Hide Caption 7 of 11 Photos: Photos: Cleveland's missing women Missing women emerge from captivity – Amanda Berry emerged from captivity in May, leading to the freedom of two other missing Cleveland women: Michelle Knight, left, was abducted at age 21 in August 2002, and Gina DeJesus, center, was 14 when she disappeared in 2004. Berry, right, had gone missing at age 16 in 2003. Hide Caption 8 of 11 Photos: Photos: Cleveland's missing women Ariel Castro arrested – Ariel Castro was charged May 8 with kidnapping the three women. He has pleaded not guilty to the 977 counts against him. Hide Caption 9 of 11 Photos: Photos: Cleveland's missing women Cleveland's notorious serial killer – In 2009, Cleveland police found the remains of 11 women at the home of Anthony Sowell. All had disappeared from across the city over a two-year period. Sowell is on death row. Hide Caption 10 of 11 Photos: Photos: Cleveland's missing women Sowell's 'house of horrors' demolished – The Cleveland home of convicted serial killer Anthony Sowell -- dubbed the "house of horrors" -- was demolished in December 2011. Hide Caption 11 of 11

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."