Marika was introduced in the flashback as a cheerful and upbeating member of Hajime's scout group. During one of their scout trip, the group became separated from the teachers and other groups and got lost in the forest. At this time, Akari Kanemoto, the other member, was bitten by a snake. Shocked by the event that transpired, Hajime hurried his friends to quickly bring Akari back to the teachers. At Hajime's suggestion, the group decided to cross the river by using a self-constructing lifeboat using their plastic bottles and shoelaces. However, Riku Kamikouji was unwilling to use his shoelaces, as his shoes was bought by his hardworking mother as a present for attending the camp. Not knowing about Riku's feeling and prioritizing Akari's safety, Marika, along with Rin Kiritani and Kotaro Inui, selflessly gave their shoelaces to Hajime and pleaded everyone to do so, with Riku reluctantly gave up his the last. Afterward, the group successfully crossed the river and rejoined with the others, safely handing Akari to the infirmary unit.
Angered at Riku for not caring about Akari's life and his selfish attitude, Marika decided to pull a prank on him by capturing a bee and put it inside Riku's water bottle, with an intention to scare him, urging Rin and Kotaro into helping her. Unknown to Marika, Riku's mother was accidently stung by the bee when she washed his water bottle and feel into a coma before passing away. Riku's father, believing his son to be responsible for his wife's death, began to physically and emotionally abuse him until he fell sick and had to stay in the hospital. Before he passed away, Riku's father apologized to him and told him truth behind his mother's death. Realizing that one of his friends was responsible for his misery, Riku visited Marika at her village's shrine to ask about what had transpired, which led to him confronting her about the prank from the past. Not being informed of situation and letting her emotion get ahead of herself, Marika came to believe that Riku still held a grudge against her and angrily confessed that she was the culprit and asked Riku about the fact that was it Rin or Kotaro snitched her out. In a confusion, Riku accidently stabbed Marika to death with a fruit knife. Shocked that he murdered Marika and believed that he couldn't turn back anymore, Riku proceed to dress Marika with a fox's bride mask and robe to disguise the crime as a curse and used the gathering of the friends who came to Marika's funeral to murder Rin and Kotaro. Because of the circumstance during the murders, Riku along with the others were excluded from the list of suspects.
However, Hajime successfully deduced the truth behind Marika's, Kotaro's, and Rin's murder, and secretly confronted Riku about his crime. Although, knowing that the three were responsible for his family's misery, Hajime was unwilling to forgive Riku for destroying their six years of friendship and killed the friends that had been through together and suggested that Riku turned himself in, adding that he will hunt Riku down should he refuse to atone for his crime, which Riku accepted. Before Riku and Hajime left with the police, Marika's father informed him a shocking truth: Marika sincerely felt bad about the prank she had done to Riku, and she was actually planning to apologize to him, had both of them didn't let their emotion get ahead of themselves. Realizing Marika's true feeling to him, Riku silently cried as they left the village and let himself be taken by the police, with Hajime being the only one who knew this painful truth as he sadly thought and wished that things didn't ended up this way.