After discovering human bones in one of the prep rooms, Shoutarou wants to finds out why Sasaki left it there.
From the looks of it, Shoutarou dragged Sakurako, who is reluctant to go to not only bring the belongings that Sasaki left back to his older sister, Haruma Sayuki, a woman who is in the wheelchair that couldn’t walk. Apparently, Natsuko was her caretaker for her taken in by their father, who passed away from an unknown illness with only a few years to live.
Apparently, Sayuki had a secret relationship with her father’s acquaintance and Natsuko wrote love letters on her behalf without realizing the person Sayuki was in love with was already married. After realizing that, Sayuki’s father planned an arranged marriage. To make matters worse, Sayuki was pregnant out-of-wedlock. To protect her from backlash, Natsuko took the blame even though it meant that she couldn’t be with Sasaki. In short, the family abandoned Sayuki for what she did along with dealing with the grief from her baby dying prematurely, which got buried inside a music box.
So, you might be wondering, what this sad and somber mystery has to do with Sakurako being reluctant to solve it. It does because it made Shoutarou realize something. He found out that the cat bones that Sasaki left was the remains of a second cat named Radius. Both Ulna and Radius are the bones that make up the forearm. Given that, both cats were close with each other. They apparently died from being poisoned by someone, which led Sakurako, who was a student at the school that Shoutaro attends to learn Osteology from Sasaki by rebuilding one of the specimens.
At the end of the day, while the story involving Sasaki’s love interest, Natsuko and his sister Sayuki was a rather tragic one, it does show some significant things about Sakurako’s dark past. The death of her two cats that were close to Sakurako enabled her to learn the art of Osteology and of course deepen her obsession over bones. In fact, the mysteries from past episodes give some subtle hints about her past such as her dead little brother. This is what makes Sakurako so mysterious aside from her obsession over bones.
Still, I’m surprised that Shoutaro was able to piece everything together by taking his observations to figure out the truth about why the cat bones disappeared along with the meaning behind the poem that Natsuko left. This is what essentially makes him useful when Sakurako decides to solve a mystery together.