Read the full summary of the episodes here.
He and Nine are first seen in Episode 1 stealing plutonium from a nuclear fuel reprocessing facility. They leave a spray-painted message at the scene, "VON." The meaning of VON is "hope" in Icelandic.
Six months after the incident, they enroll in a high school in Tokyo as transfer students. The morning of his first day of school, Twelve meets Lisa at a swimming pool, where several bullies were coercing her to jump in. He humorously jumps in himself, smiling like an idiot, to help diffuse the grimness of the situation. From that moment, he appears to take a mild interest in her that grows stronger as the series progresses.
While infrequently attending school, he and Nine initiate their plan to perform a series of bombings in the city. To foreshadow each attack, they upload a video to the internet under the organization name "Sphinx" in which he and Nine, donned in masks and introducing themselves as Sphinx #1 and Sphinx #2, leave a riddle that is supposed to lead the police to the site of the bomb. They welcome the police to play along with their game and defuse it.
Their first bombing occurs at the Tokyo Metropolitan Government Office. They strategically coordinate their bomb planting with a blackout, giving them 25 minutes to plant bombs (embedded in plush toys) throughout the building while the security cameras are down. In the act of planting bombs, Twelve encounters his classmate, Mishima Lisa. He hands her one of the plush toys and tells her not to dispose of it until he instructs her to, and leaves to meet up with Nine. Twelve tells Nine about the encounter and gives him two options: they can either kill her on the spot (and thus avoid the risk of her reporting them) or they can show mercy and save her from the explosion, only to force her to become an "accomplice." Twelve convinces Nine to go with the second, reasoning that ever since the last time they failed to save someone, Nine kept having nightmares.Twelve is implied to be keeping tabs on Lisa after she becomes their accomplice as he unexpectedly checks up on her at a bookstore, without Nine's consent (although Nine reveals that he knew Twelve had met with Lisa). Lisa follows Twelve outside on a crowded street where he warns her (with a smile) that if she makes one wrong move, he will not hesitate to kill her.
Around the time of the fourth bombing, Twelve is confirmed to be keeping tabs on Lisa's movements (using a GPS tracker on his phone). He correctly speculates that she must have run away from home, and seems concerned for her safety as he checks her progress frequently throughout the day. Nine reminds Twelve that he had agreed not to get involved with her to which Twelve replies that if she is taken into custody and talks about them, it could cause trouble for both of them. After some time passes, Twelve disobeys Nine and ends up searching for Lisa finds her sitting at a bench at night and tells her to go home. Lisa refuses, saying that she has no home to return to, but at the same time feels stupid for thinking things would change for the better if she became an accomplice. Twelve remains silent as Lisa leaves but he rescues her from two security guards before taking her away on his motorcycle. On their way to the Sphinx hideout, Lisa asks him: "Are you going to destroy the whole world?" to which Twelve laughs. Lisa laughs with him, noting that she hadn't laughed like that in "a really long time".The sixth attack was on a train, but it didn't go as well as planned. Nine and Twelve were debating on whether they should give Lisa shelter after she collapsed. Later they catch her playing with their cell phones (clueless that they were part of a bomb set up), and she faints again after learning she could have blown up the building. Nine and Twelve later place the same bomb inside a passenger train. Upon returning home, they find Lisa attempting to cook (to demonstrate her usefulness), and Twelve tries a bite before stating that the food wasn't edible. He notes that Lisa's face is red indicating she still has a fever and tells her to go rest. Despite his earlier criticism, Twelve later comments to Nine that the food had value despite its taste, since the food of 'that place' didn't taste like anything. Afterwards, Nine and Twelve send another video, in which Twelve introduces himself as Sphinx Number 2 and jokes that he is looking for a girlfriend who can make bombs and cook well (possibly making a reference to Lisa and her inability to do neither). The riddle this time was: 'Where did the punishment take place for FEZ 5889, the angel who planted a grapevine?' , also saying that an explosion will occur after 8:00 p.m. if the riddle isn't solved before then. As the day goes by, there have been no reports of the bomb being recovered, and Nine and Twelve, not wishing to kill people, plan on deactivating it using a cellphone. However, cellphone reception goes out all over the area, preventing them from doing so, forcing them to try and remove it manually. Twelve runs off to the train platform. While Nine tries to pinpoint the train's location, but Nine is horrified when his computer is hacked into. Meanwhile, Twelve is waiting for the right train with the bomb to arrive. Nine then manages to find the train using a second computer and rushes to the station and afterwards when the bomb goes off, Twelve loses contact with Nine. After waiting for a while Nine finally reacts and it seems like he is unhurt and then he and Twelve receives a text message saying, "I found you." Both suspect the sender is a girl named Five.
After the train incident, Nine has a nightmare involving Five. He and Twelve discuss her, and Nine concludes that Five is continuing a "game" the three played together previously. When Lisa offers to help them, Nine shows her pictures of the aftermath of the subway train bombing and states that this is what they were doing, which shocks her. While Lisa does the laundry on the roof, Twelve meets her and tells her that he has synesthesia, a condition that allows him to see colors through sound. He describes her voice as being a "pale yellow" which is a rare color for him to see. Lisa seems upset by Nine's disapproval of her but Twelve jokes that Nine is merely very sadistic and doesn't actually hate her making Lisa laugh. Lisa momentarily stuns Twelve when she mentions that she is a little jealous of how well Twelve knows Nine. Twelve recovers and smiles a little sadly and replies that he and Nine have been together for a long time.
Later on Five sends a text containing a riddle to the public: 'GIGFHEEGMHJL FD. Gaius Julius Ceasar gone to Arab. Sphinx.', posing as Sphinx; the riddle can only be decoded via Caesar cipher. The answer turns out to be a set of coordinates leading to Haneda Airport's International Terminal, one of the locations targeted by Nine and Twelve. Knowing that they will be caught by Five while trying to enter the terminal, Nine reluctantly decides to bring Lisa along with them, since she isn't known to be associated with him and Twelve. Upon arriving at the terminal, Nine realizes that Five plans to play a game of chess using the airport as the chessboard.Nine and Twelve decide to play along with Five's chess game. When they get closer to the bomb's potential location, Twelve sends in Lisa to create a distraction; after some hesitation, she sets off flares that trigger a fire alarm. Twelve then hacks into the camera system and brings the recordings back by five minutes, allowing Nine to rush to Five's location without being detected. Afterwards Nine and Twelve regroup again. Then they receives a phone call from Lisa describing her and the bomb's whereabouts: a plane that is on autopilot and headed for a terminal filled with numerous commuters. Realizing there is a way to save both Lisa and the commuters, Nine contacts Shibazaki, convinces him that the latest bomb isn't Sphinx's, and instructs him to infiltrate the control tower and take control of the autopilot system. Nine and Twelve rescue Lisa from the plane. The plane explodes without injuring anyone, and Shibazaki spots a masked Nine, who signals him in thanks before leaving.