Yotaro Osato is a protagonist of the Matchmaker story within The 25th Ward: The Silver Case. He is the partner to Shinkai Tsuki in the Regional Adjustment Bureau. He acts as not only a foil to Tsuki's hardened personality, but also as somewhat of a Mcguffin in the Matchmaker portion, as his past is a mystery with his acceptional talents being used by the Adjustment Bureau.
For the first few chapters of the Matchmaker story, Osato is shown as a somewhat carefree and strange individual. He makes brash decisions, and is very dumb at times, making the wrong decisions leaving it up to Tsuki to pick up the slack. This is most obviously demonstrated when he accidentally kills a member of the Postal Federation due to him misunderstanding what Tsuki meant when he asked him to "escort him", , thinking that meant "escort him to the next life".
Osato through the course of the story develops a more docile personality as he learns from his mistakes in the first chapter and proves to be quite helpful, as he tends to be more tech savvy than Tsuki. He is shown to have profound knowledge about the "Catherine" device and its newest iteration known as the "Catherine Nano".
In the last two chapters it is revealed that he had lost his memories of his origins and that he harbors a more sinister personality. It is revealed that not only is he the son of Sundance Shot, but he is also a catalyst to a special material full of "criminal power". Supposedly this criminal power is not just how willing a person is to commit a crime, but a disease like energy that no matter the individual, it will change the user into a "war machine". Osato only shows a little bit of this by killing Tsuki's former mentor, Ishiki, and revealing his silver cat-like eyes before being drugged and sent to a lab.
After being rescued by Tsuki from Shigino, after being kidnapped by him from the lab, it is shown that he was being used as a supply for criminal power, being hooked up to a machine that could drain his essence. Shigino had already placed the machines needle into his neck, dooming him to bleed out even his none of the energy is drained. When he reawakens as his normal self, he realizes that he is a threat and asks of Tsuki to "adjust" him. He did not want to suffer the pain of bleeding our and wanted Tsuki, his mentor, to help him end the pain. After Tsuki ends Osatos life, he and the other remaining members of the Adjustment Bureau hide his body from the 25th ward in order to keep his "criminal power" from being misused even after death.