Shaun (synth)

S9-23, also known as Shaun and as synth child is a prototype synth created by the real Shaun, and lives in The Institute until the Sole Survivor arrives.

Background
Shaun was created by the director of The Institute, Father, or the real Shaun, and is one of the human-like synths, built using the real Shaun's DNA as a base of the synth child's (and other human-like synths) organics. He was 'brought up' and programmed by Father.

When the Sole Survivor arrives to The Institute to locate their child, they are briefly led to believe that the synth Shaun is the real one in order to test the child's responses. Synth Shaun responds negatively and calls for help from Father when approached by the Sole Survivor.

Other interactions
If he is not left behind in the Institute, after the ending, Shaun will ask the player character to bring items (i.e. a hot plate) so he can 'take it apart' and in return will build useful items for the player character, including the Wazer Wifle. Additionally, the player character can ask Shaun to live with them, and can send him to a number of locations, similar to a companion.

If the player completes "Nuclear Family" option, Shaun will be found and interacted with in the Advanced Systems Lab. The player has the ability to choose where Shaun will live by talking to him, second time he must be moved via workshop. Shaun however, will build the useful items if you bring him junk.

Effect of player's actions
If Shaun is not left behind in the Institute after the Nuclear option is chosen, upon meeting with the player character again and being interacted with, he will give them a holodisk named Shaun's holotape as instructed by Father, saying "I didn't listen to it, so I dunno what it says, but I think it's important." The holotape, recorded by Father, reveals that he re-programmed Shaun to believe that he was the son of the player character, and pleads with the player character, saying "I would ask only that you give him a chance. A chance to be a part of whatever future awaits the Commonwealth."

However, if the player chooses the Nuclear Family option, they can still obtain the holodisk, but Father's speech will differ, still revealing that he reprogrammed Shaun and saying "Both he and you deserved a chance to... To be a family. Please, take care of him."

Should the Sole Survivor leave Shaun in the institute following the assault by The Brotherhood of Steel or The Railroad, Shaun will never again be seen in the game, likely dead/destroyed.

Appearances
Shaun appears only in Fallout 4.