Description from original post (created 2016-05-18 13:02):
An unusual robotic detective, Nick Valentine operates a small agency in Diamond City. In addition to possessing the memories of a late pre-war detective, Nick has certain synth gifts that complement his investigative skills: he is very effective at hacking computers, and adept at both ranged and melee combat. Valentine believes himself to be a prototype between second generation synths and the latest, which might explain why he exhibits sapient intelligence and is not innately hostile towards non-Institute humans. He is generally well respected throughout Diamond City despite his mysterious origins.
Although Diamond City holds no small amount of hatred for the Institute and their synths, Nick gained their respect by saving the daughter of the city's former mayor when she ran away with a caravan trader. The caravan trader and his companions were, unbeknownst to the girl, actually kidnappers who (as kidnappers tend to do) proceeded to take her hostage. Nick stumbled upon the kidnappers purely by happenstance when he was wandering the Commonwealth wasteland, having been almost exclusively itinerant since his "awakening". Finding himself held at gunpoint and likely the girl's only hope, Nick scared the abductors away by claiming that he had a bomb inside him and beeping (true to Fallout-style humor, Nick claims that he literally just said the word "beep" repeatedly).
When the Mayor's daughter told Nick what had happened he returned her to her father, who in turn gave him a house in Diamond City as compensation despite the citizens' dismay and objections; Nick gradually assuaged the citizens' fears by acting as Diamond City's handyman. His formidable detective skills were brought to the forefront when he helped to find a wife who was thought dead: Nick discovered that she had in actuality run away with her lover. In time, people stopped asking Nick to fix things when it became clear that he had found his true calling as Diamond City's private investigator.
Once the Sole Survivor gains a sufficiently positive relationship with Nick, he will reveal that his personality and memories are those of a pre-war detective who volunteered to have his memories encoded into an early synth model by the Institute (which was at the time still known as the Commonwealth Institute of Technology). Upon awakening in his synth body many years later, Nick -- being possessed only of those memories and with no recollection of his construction nor of the Institute -- suffered tremendously from confusion and disorientation as he attempted to navigate the Commonwealth and to acclimate himself to his new reality.
As synths and their associated technology were largely unknown at that time, most wastelanders regarded Nick with equal parts fear and awe; however, he was eventually accepted into a small local community where a mechanic offered to repair the damage he had sustained in his travels. During this time Nick befriended a child he knew only as "Jim"; the pair formed a strong relationship. Nick eventually left that settlement; upon visiting years later, he found that it had been pillaged by raiders. The fate of its population remains unclear.
During the quest Long Time Coming Nick reveals that an old criminal from before the war, Edward Winter, killed his fiancé Jennifer Lands. After killing Eddie, Nick will lead you to a spot nearby which he then explains is the spot where Jennifer was murdered.