Kaoru Kobayashi was born on November 30, 1968, in Sumiyoshi-ku, a ward in Osaka, Japan. He grew up in a poor family, which affected his early life. To help support his family, he worked as a paperboy during his childhood. Tragically, his mother passed away in 1978 when he was just a child.

As he got older, Kobayashi's behavior became concerning. In 1989, he was convicted of sexually assaulting eight children. As a result, he received a suspended prison sentence of two years. His actions did not stop there. In October 1991, he attempted to kill a five-year-old girl and was sentenced to three years in prison. He was paroled on November 9, 1995, and officially released on July 23, 1996.

After his release, Kobayashi found work at a newsstand for The Asahi Shimbun in Tomio, a neighborhood in Nara Prefecture. He worked there from March to July 2000. Later, he was employed as a newspaper deliveryman for the Mainichi Shimbun in the Ikoma District of Nara Prefecture. He lived near Tomio at that time.

On November 17, 2004, Kaoru Kobayashi kidnapped a seven-year-old girl named Kaede Ariyama. She was on her way home from Tomio North Elementary School in Nara, Japan. Kobayashi used Ariyama's cell phone to send a picture to her mother with the message, "I've got your daughter." Later that day, Ariyama's body was found in Heguri, Nara Prefecture. The autopsy showed that she had drowned, with evidence suggesting she was drowned in a sink or bathtub. Various injuries were noted on her body, and it looked like Kobayashi had undressed her before killing her and then dressed her again afterwards.

On December 14, 2004, Kobayashi sent another email from Ariyama's phone to her mother. In this message, he threatened to take Ariyama's baby sister next. He also displayed Ariyama's photo at a local bar, telling customers he found it online.

Kobayashi was arrested on December 30, 2004. His arrest was made easier because he had sent a message from Ariyama's phone to his own, which was tracked by local cell phone towers. When police searched his home, they found Ariyama's phone, a backpack belonging to her, and other disturbing items including child pornography and stolen girls' underwear.

Witnesses claimed that Ariyama was seen walking to Kobayashi’s car, which implied they might have known each other. However, Kobayashi insisted he would have kidnapped anyone. On January 19, 2005, he was formally prosecuted for kidnapping.

Kobayashi's earlier criminal history, including sexual offenses against girls, drew public attention to potential changes in laws regarding sex offenders in Japan. His case sparked discussions about the safety of children and how to handle repeat offenders in society.

His trial began on April 18, 2005. He expressed a wish to be quickly sentenced to death and aimed to be remembered comparably to other notorious murderers. During the trial, experts determined he suffered from antisocial personality disorder and pedophilia but was still capable of understanding his actions.

On September 26, 2006, Kobayashi was sentenced to death by hanging. He initially appealed but later changed his mind and withdrew the appeal. His legal team later claimed this withdrawal was invalid, but courts upheld the original death sentence.

After various appeals and legal processes, Kobayashi's death sentence was confirmed by the Supreme Court of Japan. He was executed by hanging on February 21, 2013.