Shun was born in Tokyo, Japan in 2018 to an American mother and Japanese father. In this reality, the earth is in a steady decline due to pollution, overuse and subsequent scarcity of its resources, and several other factors that has led for a more serious drive to colonize another planet. Shun lost both his parents early on in life. It is implied that his family - specifically, his father - had ties to the yakuza that ended up being the cause for at least his death. As a result of this fractured family at such a young age, him and his younger brother ended up taking wildly diverging paths in life.

While his brother Takeshi joined law enforcement and became a police officer, Shun ended up falling to the same web that his father before him presumably had. He found a family again in the yakuza. Despite his brother's attempts to likely dissuade this, Shun had believed from a young age that it was likely the path he would take. This is something he actually relays to Lana aboard the Origin later; he recalls a story of trying to protect his brother from bullies only to be the one Takeshi seemed most scared of after assaulting the other boys with a baseball bat.

Even for the ways he feels like it was inevitable, he doesn't actually enjoy the life that he's made for himself. There was simply no other choice. After years of involvement with the yakuza, he knows that it's impossible to remove himself from, as he says, the "shit he's entrenched in." That's when Takeshi comes to him with a proposition: there is a company named Siren that's looking for people to colonize a new planet by the name of Thea. They offer to anyone that volunteers to aid in this mission what they refer to as Tabula Rasa or a blank slate. No matter what anyone's record, it will be wiped clean in order to begin a new life on Thea.

He agrees to go... on the condition that his brother goes with him. They'll leave earth together or not at all. His brother declines at first but ends up changing his mind when the yakuza are cornered by police. Takeshi is able to warn Shun so that he can get out but not unscathed. Shun is shot and Takeshi comes to help him. Unfortunately, when they arrive back at Takeshi's apartment the yakuza boss, Oyaji, is waiting for both of them. He considers their being caught in the first place Shun's fault for his connection to the law via his brother. In order to punish him for this perceived betrayal, Oyaji shoots and kills his brother on the spot.

Afterward, Shun is consumed by a desire for vengeance that burns him alive from the inside. He knows that he can't directly retaliate against Oyaji. Instead, he ends up tracking down the man's father and threatening his life. When Oyaji shows up to save his father, Shun raises a gun to shoot and kill him but ultimately can't go through with it. It ends up being one of his fellow yakuza members, a man named Murakawa, that betrays Oyaji and shoots him dead. Now free from the yakuza's grasp, Shun ends up leaving earth and joining Siren's cause to colonize Thea in exchange for a chance to start anew.

Of course, he ends up out of the frying pan and right into the fire. He and other members aboard the Origin are awoken nine days before arriving to Thea. They discover that the ship was evacuated and they were left behind. The reason why? A large meteor struck the ship. Upon investigating, they come across Rey and Taylor who are seemingly the only original crew members left behind.

After talking with Rey and a close encounter with a deranged Taylor, they are able to piece together that the meteor houses a parasitic alien life form that infects humans. The process is extremely painful when it doesn't work and ends up killing the host in grotesque ways. But when it does work, the parasite latches onto someone's brain in order to control the person and try to mimic normal human behavior to remain undetected. It seems to have a strong sense of self-preservation and does this because otherwise, it will die.

( ***warning for major show spoilers after this point*** )

However, the alien's attachment to the brain makes people lose their memory. Unfortunately, being strangers forced to work together no one knows anything about anyone. Once it's discovered that one of the aliens has escaped and infected one of their own it breeds mistrust and fear among them. They must work together while simultaneously keeping each other at a distance to try and discover who is infected.

In the last episode, I Am, Shun discovers a photograph on Taylor's body of him and his brother Sam that Rey left behind. He recalls earlier in the season that Lana had told him about her brother, also named Sam, and goes to investigate. In Taylor's old room, the one that Lana has inhabited during the show, he finds a video recording from Taylor talking about him missing home where he and his brother Sam would jump into the lake outside their home on hot summer days. This is word for word what Lana had told him previously and he starts to come to the realization that Lana, the only person aboard he thought he could truly trust, is the one among them infected and the parasite was using Taylor's memories and not Lana's own.

He finds a tape recording after of Lana detailing her own life that confirms his suspicions. In the recording, she says that her name is Lana, the names of the other passengers, and that she is the only known to be still alive of the parasites. Unfortunately, Lana discovers Shun unearthing these pieces of evidence and she attacks him. In winning the fight, Shun raises a gun to kill Lana but finds that he can't after she/the parasite say to kill her. It recites back to him something they'd talked about earlier, that it finally realizes what he meant when he said there are "worse things than dying."

Shun ends up knocking Lana/the parasite out and putting her aboard a Siren ship that had attached itself to the Origin (and could only carry two people) and setting it to return to Thea presumably for Siren to deal with. He is seen forlornly watching the ship head to Thea, feeling betrayed and saddened by the only person he allowed himself to trust being the one that was infected almost all along.