(NEW YORK) -- Caden Leutner remembers the day police came to his family's Waukesha, Wisconsin, home to deliver the horrific news.
It was May 31, 2014 and he was only 10 years old at the time.
"I was on the couch and then the detectives knocked on the door and they said, 'Where's your mom?'" Caden Leutner said in his first exclusive interview with ABC News. "I said, 'Oh, she's in the back.' And she ended up coming in and saying, 'Caden, we have to go to the hospital.'"
His 12-year-old sister, Payton Leutner, had been stabbed. He and his mother, Stacie Leutner, rushed to the hospital to be at her side.
It would be a while before the family would learn the details of the assault on the sixth-grader: that Payton Leutner's friends Morgan Geyser and Anissa Weier had tried to kill her to please a fictional online character known as "Slender Man."
While Stacie Leutner went with doctors to see her daughter, Caden Leutner stayed in a waiting room with Waukesha police Officer Dan Klein, the first police officer to respond to the scene of his sister's stabbing.
"I think Caden was very curious," Klein said. "Why is he with police officers? Why are they talking to his parents and where is his sister? He tried to listen a little bit at times but I kept trying to redirect him."
Klein stayed with Caden Leutner for hours while his parents spoke to doctors and tried to get answers.
"It was rough but I do want to thank Officer Klein for sitting with me in the hospital," said Caden Leutner, now 15. "I think he's a really, really nice person and he deserves a lot more (credit) than he gets."
To keep the 10-year-old's mind off the frightening circumstances, Klein said, he talked with the boy about the things Caden Leutner liked.
"I remember he was in fourth grade at the time," Klein said. "Caden was very much into fishing and all he could talk about was his upcoming talent show."
"We talked about fishing, board games, just a ton of different stuff and I think it was amazing what he did," Caden Leutner said.
It was the beginning of a long road to recovery, not just for Payton Leutner but for her entire family. As she healed from her physical wounds in the hospital, Joe and Stacie Leutner tried to make sure their 10-year-old son never felt overshadowed.
"We were in the hospital with her and we made time to go and be with him every single day," said Joe Leutner, the children's father. "One of us picked him up from school every single day ... to spend a couple minutes with him to make sure that he wasn't forgotten. But it wasn't enough, and I know it wasn't enough."
While Payton Leutner was in the hospital, extended family took care of her younger brother.
"It was a very scary experience for him, and he had to go through that with being watched by family and not the parents," Joe Leutner said. "He struggled. He rebelled. He fought because he was hurting. ... Just because it happened to Payton doesn't mean that it didn't happen to him as well. That was his sister. He was 10. He was confused. He was scared."
When Caden Leutner finally learned the details of his sister's attack, he said, he struggled as a 10-year-old to comprehend it.
"I couldn't believe it. I couldn't believe that that kind of thing could have happened," Caden Leutner said. "You hear about that happening to people and you think, 'That could never happen in my family. That could never happen to me. My sister, my mom, my dad. That would never happen to me. We live in a great neighborhood. We are around great people.' But there's always a chance that that could happen."
He also acknowledged that as many people learned about Payton Leutner's story, he felt as if he was on the outside of the circumstances.
"Not many people knew about me and I felt kind of removed from the situation," he said. "I felt like no one really understood my side of it and no one really understood what I was going through."
Stacie Leutner said she also felt changed by what happened to her daughter and its impact on her family. But, she said, she now feels that the changes she and her family have gone through have been for the better. After weathering five years of healing, litigation and media coverage, Stacie and Joe Leutner are no longer married, but say their message will always be one of solidarity for their children.
"We will always be there for them no matter what. ... We will always do what we need to do for them and we'll do it together because they are the most important thing to us," Stacie Leutner said.
Geyser and Weier were charged in adult court with first-degree attempted intentional homicide after the attack. Weier pleaded guilty to a lesser charge and was found by a jury to be not guilty by mental disease or defect in 2017. Geyser pleaded guilty to the first-degree charges against her and in 2018, as a part of her plea agreement with prosecutors, was convicted but found not guilty by reason of mental disease or defect. Weier and Geyser each were sentenced to up to 25 years and up to 40 years, respectively, in a mental health institution.
Both of the Leutner children have made incredible strides in the five years since the attack, Joe Leutner said.
In a full-circle moment for Caden Leutner, when he entered high school, Klein, the police officer who had kept him company on the day his sister was attacked, was the school's resource officer.
"He would check in with me whenever I'd walk past and he'd say 'Hi,'" Caden Leutner said. "He makes sure to remind me, 'You can come by whenever you want. Stop in my office if you want to talk.' So, he's a really very nice person."
Now a sophomore in high school, Caden Leutner said that he'd learned a valuable lesson after experiencing what happened to his family.
"It taught me to respect others and to always realize that there's other people going through stuff and you want to help those people even if it's not happening to you," he said.
He said he has also picked up important lessons from his sister.
"She's taught me how to be humble. She's taught me how to laugh at different things," he said.
Laughter has also returned to her family's home, Stacie Leutner said. In 2014, Stacie and Joe Leutner told ABC News that they'd feared Payton Leutner's smile wouldn't return after the attack.
"Caden is our court jester," Stacie Leutner said. "He keeps the family laughing, but she smiles, and laughs, and has a great time. (Payton's smile is) definitely back."
Leutner family spokesman Steve Lyons, of SJL Government Affairs and Communications, said the family is very strong.
"It's really fun to be part of that wonderful journey, to watch it at a very happy ending," Lyons told ABC News.
Looking back, Caden Leutner said he feels it is his parents who are owed a thank you for helping him and his sister come out on the other side of the ordeal with grace.
"They are great, great parents," he said. "They were able to teach me and my sister so much about being humble, modest. ... They were able to teach us that. They are fantastic parents."
Watch the full story with ABC's David Muir on "20/20" Friday, Oct. 25 at 9 p.m. ET on ABC
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