The character known as Cop in Death Note, whose full name is Soichiro Yagami, is the chief of the National Police Agency's Criminal Investigation Department. As a high-ranking officer, he is trusted by the national police and is chosen to lead the Japanese Kira Investigation Headquarters after the detective L determines that the serial killer known as Kira is likely operating from within Japan.

Soichiro is defined by his powerful and unwavering sense of justice. He is a kind, honest, and responsible individual who follows rules and regulations strictly in his pursuit of justice. The creators of the series have described him as the only true symbol of justice in the story. He is a serious man who gains the respect of his colleagues and subordinates. His sense of duty is so strong that he refuses to give up on the Kira investigation even when the police force is pressured to withdraw, choosing to continue the fight at great personal and professional risk. Unlike many other characters, Soichiro never displays corrupt or anti-heroic tendencies; his actions are solely driven by a desire to uphold the law. He firmly believes that no matter what the purpose may be, killing people can never be the answer to anything and that any happiness built on the deaths of others is not true happiness.

His primary motivation is to arrest Kira and stop the mass murders occurring across the globe. This goal is driven by his professional duty as a police officer and his personal moral code. However, he is equally motivated by his role as a father. He cares deeply for his family, including his son Light and daughter Sayu. This creates a profound internal conflict, as he is determined to prove his son's innocence when L begins to suspect Light of being Kira. He is in a desperate situation, forced to agree to extreme measures like having his own son confined and monitored to clear the family name and find the truth.

In the story, Soichiro serves as the leader of the Japanese Task Force and the public face of the police investigation. He is one of the few people who stands up to L, calling him a hypocrite for using a death row inmate as bait. His role is to bridge the gap between the unconventional, secretive methods of L and the official police procedures. He participates directly in critical moments of the investigation, such as personally ramming a police vehicle into a television station to stop a broadcast from the second Kira. After L's death, Soichiro remains a key member of the task force, which is now led by his son, still oblivious to Light’s true identity.

Key relationships define his arc. His relationship with his son, Light, is the most tragic element of his story. He loves Light immensely and trusts him completely, which is exploited until the very end. His bond with L is that of a reluctant but respectful partner, as he often disapproves of L's manipulative tactics but recognizes his genius. He also acts as a mentor figure to younger officers like Touta Matsuda, inspiring them with his dedication.

Soichiro undergoes significant development, culminating in a tragic sacrifice. When his daughter Sayu is kidnapped by the mafia leader Mello, he is forced to choose between his duty as an officer not to hand over the deadly Death Note and his duty as a father to save his child. He ultimately chooses his daughter. To retrieve the notebook from the mafia, he makes a deal with the shinigami Ryuk for the Shinigami Eyes, which allows him to see a person's name and lifespan by looking at their face, at the cost of half of his remaining lifespan. In the ensuing raid on the mafia's hideout, he is fatally shot. As he lies dying, he uses his new ability to see his son Light's lifespan, and because Light is not a Death Note owner at that moment, his lifespan is visible. Soichiro dies with a sense of relief and happiness, believing his son is not Kira.

His notable abilities include strong reasoning and analytical skills. While not on the level of L or Light, he is capable of picking up on key clues that advance the investigation. His most prominent ability, however, is his immense mental fortitude and willpower. He is able to withstand tremendous stress and legal dilemmas for the sake of justice, even having himself imprisoned to maintain the integrity of the investigation. His final tactical ability is obtaining the Shinigami Eyes, which gives him a supernatural power to see names, though he is ultimately unable to use it to kill Mello before being mortally wounded.