Sakunosuke Oda/Synopsis

The Untold Origins of the Detective Agency
Sakunosuke was hired to supposedly kill a certain lady, a company president, but was framed up by the president's secretary.

He was captured and put in a room adjacent to the office with both his hands and legs tied to a chair and a sack over his head. His belongings are laid out in a table behind him. Yukichi Fukuzawa was the president's hired bodyguard, and, after taking the knowledge that his client had died by being pushed off her office window, he came to see the scene of the crime and the captured assassin. Fukuzawa decides to test just how good Sakunosuke as an assassin. Picking up a pen from Sakunosuke's belongings, he sends out killing intent together with quick-drawing the pen (like a sword) at him. Sakunosuke successfully dodges even without seeing what Fukuzawa was doing by moving sideways, making himself topple on the floor, still tied on the chair; this impresses Fukuzawa and he makes the deduction that Sakunosuke is no ordinary assassin, and must have been paid a high sum of money.

After Ranpo Edogawa comes in the office looking for employment and ends up revealing bit by bit to Fukuzawa how the secretary did the murder, Sakunosuke makes his move and undoes the binds on his legs, enabling him to move around. He and Fukuzawa had a short crossing of blows, but Sakunosuke eventually successfully gets a hold of his gun. At that moment, Fukuzawa realizes who his opponent was, but before he can stop Sakunosuke, he has shot the secretary dead—all that with a sack over his head and his hands still tied behind him. Fukuzawa was distracted for a moment with the dead secretary, and that was when Sakunosuke made his escape.

Dark Era Arc
Sakunosuke was first seen drinking in Bar Lupin with Dazai and Ango. The three of them converse like old friends, an odd sight considering their respective positions in the mafia—Dazai, one of the five executives; Ango, an intelligence member; and him, a low ranking member who is only assigned the most trivial of tasks due to his refusal to kill. After some idle chatter, they say goodbye for the night, but not before Dazai convinces Ango to take a photo of the three of them to commemorate their friendship.

He was called out by Mori a few days later. The leader of port mafia informs him that Ango had gone missing ever since that night, and tasks Oda with tracking the man down. Though confused at Ougai's choice of assigning him that mission, he nonetheless sets out to investigate Ango's room. During his investigation, he was assaulted by members of Mimic, an organization that is seeking to wage war against Port Mafia and discovers an old gun hidden in Ango's room - the same model that identifies Mimic members. Dazai also comments that Ango was lying about what he did on the day the three of them met up at a bar - further increasing Oda's suspicions.

Distraught at the possibility of Ango being a double agent for Mimic, he goes to visit the five orphans he adopted. Seeing them happy and well reassures Oda a little, and reaffirms his decision to leave the mafia as soon as the children were grown enough to support themselves.

Later, he receives a call from Dazai. The Mafia had discovered new clues about where the Mimic members may be hiding, and Dazai urges Oda to go and watch the location first. However, Oda decides that he wanted to investigate the building before Dazai and his subordinates came, in fear of them finding more damning evidence against Ango. He founds Ango in the building, bound to a chair next to a time bomb. Oda hauls Ango out of the building and barely manages to escape the explosion unscathed. At this point, Oda had figured out that Ango wasn't a double agent for Mimic - but a double agent for the Mafia. He disappeared because his status as a spy was revealed to Mimic. Ango tells Oda some information about Mimic's commander - a man named Gide - but soon left with a group of black-uniformed men that were neither Mimic nor the mafia (later revealed to be part of the Japanese ministry's supernatural division). Oda was not able to give chase as he triggered a slow-acting poison trap that knocked him out.

He wakes up in a hospital room, having been recovered by Dazai's subordinates when they reached the building. Dazai informs him that several Mafia members (including Akutagawa) are in open conflict with Mimic members in front of an art museum. Oda is quick to rush to their aid and saves Akutagawa from being killed by Gide- the commander of Mimic who shares Oda's ability to see a few seconds into the future.

Delighted to find someone who shared his ability, Gide tries to provoke Oda into fighting and eventually killing him. Gide and his organizations were former soldiers betrayed by their own country, and are now seeking to die amid the thrill of battle. However, Oda refused to kill Gide due to his personal beliefs. Gide retreated in fury, but promised to "make Oda understand him".

After the battle, Dazai takes Oda to a place where they could find Ango - the bar that the three of them used to frequent. Dazai had correctly deduced Ango was neither a spy for the mafia or Mimic - rather, he was a triple agent sent by the Japanese Home Affairs Ministry's Special Division for Unusual Powers to keep an eye on the port mafia. Ango divulges some intelligence regarding Mimic to Dazai and Oda, sharing one last drink with the two of them while lamenting their lost friendship. With Ango's identity revealed, there was no possibility of the three of them staying friends with the existing conflicts of interest. The atmosphere was somber as Ango left for good, leaving behind the photo they took the last time they were together.

Afterward, Dazai tightens the security around Oda, as well as the people close to him, including hiding away his five adopted children in a mafia safe house. However, that was all for naught as the Mimic members managed to kidnap the children and kill them all in an explosion right before Oda's eyes. Pushed over the brink of despair, Oda decides to go back on his principle of not killing and take revenge on Mimic. Later on, this was all revealed to be part of Mori's plot (he was the one who told Mimic the location of Dazai's safe house) to provoke Oda, the only person who could counter Gide's future sight, into destroying Mimic.

Oda killed for the first time in years, taking down the vast majority of Mimic's remaining forces by himself. He proceeds to engage Gide in a fierce battle that resulted in both of them killing each other but not before reaching a level of understanding because of their similar abilities. The two of them reveal to each other their pasts and what drove them so far, and though the battle ended up with both of them dying, they earned each other's respect and experienced a sense of satisfaction after the battle.

Dazai, who had at that point figured out Ougai's entire plan, hurried to Mimic's base to save Oda. However, he only made it in time to witness Oda's final moments. With his last words, Oda tells Dazai what he had understood of the man - that Dazai will never be able to find anything to bury his loneliness, nor anything that could exceed his expectations. But if neither justice nor evil made a difference to him, Oda wanted Dazai to be on the side that saves people - something that he set out to achieve before his life was cut short. Oda dies with a smile, knowing that he'll soon see the five children he adopted again.