|人造人間のショーン については、ショーン（人造人間） をご覧ください。|
At that time, the year 2227, the Institute had made great strides in synth production. But it was never enough. Scientific curiosity and the goal of perfection drove them ever onward. What they wanted was... the perfect machine. So they followed the best example thus far - the human being. Walking, talking, fully articulate... Capable of anything.”
2077年10月の時点で、ショーンはまだ0歳であった。10月23日、大戦争が勃発し、連邦に落下しようとしていた核爆弾から逃れるため、両親はショーンを連れて最寄りのVaultであるVault 111へと逃げ込んだ。Vaultの真の目的を知ることなく、ショーンらは冷却ポットで冷凍させられた。冷凍からおよそ6ヶ月後、Vault 111内で反乱が起こり、冷凍された人々はその事を知らないまま、職員はVault 111から逃亡してしまった。
2227年、インスティチュートはConrad KelloggをVault 111に派遣しショーンを捜索させた。2人のインスティチュートの技術者と共に、ケロッグはショーンらを解凍した。親はケロッグがショーンを連れ去ろうとするのに抵抗した。その後、ケロッグは速やかにショーンの両親のうちの一人を撃ち殺し、もう一人を再び冷凍保存させ、他に冷凍保存させられた者の生命維持を解除した。生き残った親は、後にVault 111の唯一の生存者として知られるようになり、2287年にVaultから脱出することになる。
Shaun was then raised by the Institute as one of their own, later nicknamed "Father" for his role in providing the DNA required to complete the third generation synths. In time, he became the acting Director of the Institute. The Sole Survivor can find Shaun and can choose whether or not to side with him.
Shaun is convinced that the Institute is the only hope for humanity and tries to subtly convince the Sole Survivor of this. He gives them nearly unlimited access to the Institute, introduces them to the personnel of the Institute and even trusts them with missions to seek out and retrieve rogue synths. His feelings about the Institute as the future and last hope of humanity are mildly fanatical; he possesses an irrational fear of the surface and the potential threats of the Commonwealth to the Institute. Ultimately, Shaun wishes to take humanity further underground and away from the radiation, mutant and danger infested surface world.
However, Shaun does not require the rest of the Institute to adhere to his personal beliefs. In contrast to his contempt towards people above, he is not only open to recruitment of talented individuals, but allows them to gain high positions, like with Madison Li. He also does not seem to mind if his parent disagrees with his stances - as long as they don't endanger the Institute - and continues supporting them as their heir for as long as they are loyal to the Institute. Should the Sole Survivor, during the quest Nuclear Family, tell them that they don't believe that they can take his place, Shaun encourages them to create their own place rather than follow in his footsteps.
Having been raised in and by the Institute rather than by his parents, Shaun mostly lacks compassion - though he has a strong desire for humanity to survive - and seems indifferent upon remembering the death of one of his parents. He refers to the "incident" as "collateral damage" which potentially angers the Sole Survivor, though Shaun does offer that he has had his whole life to come to terms with that information. However despite this indifference he never let go of hatred towards his parent's murderer, Conrad Kellogg, whom he does not forgive even if the Sole Survivor does. Interestingly he does occasionally show some compassion, for instance he apologizes to his surviving parent for Railroad's destruction, which he does even if the Sole Survivor destroys them before Shaun orders it.
He also reveals it was his personal choice to awaken the Sole Survivor from cryostasis, not an error of Vault 111. He struggles somewhat with being reunited with the Sole Survivor in the Institute, ironically seeing one of his parents 'younger' than he is and wondering what they could have shared if the circumstances had been different.
It is later revealed that Shaun has a very aggressive form of cancer and one of the endings is for the Sole Survivor to take Shaun's place as Director of The Institute upon his death, which was his primary wish all along.
During the Minutemen/Brotherhood of Steel/Railroad endings when the player character can talk to Shaun on his deathbed; he remains convinced to the end that the player character is making a terrible mistake to destroy the Institute and is forthcoming in his bitterness at the turn of events. His parent can convince him to grant the access codes to at least partially disable some of the synth defenders fighting the player character's allied forces, assuming they pass the charisma checks, before he asks the player character to leave him be as his death is coming regardless of whether or not the Institute detonates. If this ending is taken and the young Synth is taken with them they will receive a holodisk recorded by Father and given to them by the synth Shaun after being interacted with. The cybernetic Shaun tells the Sole Survivor "I didn't listen to it, so I dunno what it says, but I think it's important".
It is revealed that the young boy was reprogrammed to believe that the player character was their parent, as the first meeting between the synth and player character (who was led to believe that the synth was their missing son) caused the synth to become confused and call for help. The reason for him being reprogrammed is explained by the real Shaun in the holodisk, saying "I would ask only that you give him a chance. A chance to be a part of whatever future awaits the Commonwealth."
- Shaun's crib mobile plays the Baby Steps theme from Fallout 3.
- According to Todd Howard, Shaun's appearance will alter based on the couple's appearance, similar to how James' appearance alters slightly depending on the Lone Wanderer's race in Fallout 3. Todd Howard's comments were originally assumed to refer to the infant version of Shaun, as they were made public at the game's E3 demonstration, before the adult Shaun was even known to be a character. In truth, only the skin color and eye color of the infant version of Shaun will change to match his parents. The adult version of Shaun encountered in the Institute will have the same hairstyle, beard, and hair color, but a skin tone that amalgams both parents at initial character creation, eye color that is altered based on the player character, and he will usually have similar eyebrows and skull structure to the female player character as set during initial character creation.
- A copy of "You're SPECIAL!" can be found in Shaun's room before and after the war lying next to his crib. It can be collected by the Sole Survivor after the war and allows player character's choice of 1 point in any one SPECIAL stat.
- Certain dialogue options between the Sole Survivor and their spouse reveals that Shaun was conceived after the two had sex at a park.
- If the Institute ending is chosen, a grave marker will be seen in the Institute's main room in front of the Robotics section. The Sole Survivor will then say "Goodbye Shaun" if interacted with.
- Shaun can be damaged without him turning hostile.
- Shaun can also be killed right after meeting him; though doing so will lock one out from the Institute storyline, making the faction permanently hostile
Shaun appears only in Fallout 4.