In the novel Edit
Nothing is known about Nick's life prior to the rise of Gilead, or even after that. All that is really known about him is that he became a Guardian and was assigned to the household of Commander Fred Waterford. Nick lives in the household with the Waterfords over the garage. Offred suspects that Nick is an Eye because she believed he was too casual for a Guardian.
Offred finds Nick attractive. He is described by Offred to have 'a French face, lean, whimsical, all planes and angles, with creases around the mouth where he smiles.' The night after the Ceremony, Offred slips out of bed. Nick approaches her in the sitting room, alone, and speaks with her, which is forbidden. They shared a moment but did not last because both thought it was too risky. Nick told Offred that the Commander wanted to see her the following night. Nick escorts her to see the Commander in his study, the Commander trusts his silence and drives them to Jezebel's.
After Offred's night at Jezebel's, Serena Joy secretly arranges for Offred to visit Nick in his rooms and sleep with him to conceive a child. Offred sleeps with Nick willingly. She quickly develops feelings for him and he seems to reciprocate. Offred opens up to him, telling him a great deal about herself and her life prior to Gilead. Towards the end of the novel, Offred believes she's pregnant and tells Nick, as the baby is almost certainly his. Nick seems happy and saddened by the news as he will not be able to play an active role in the child's life.
When the Eyes come for Offred, Nick comes to her room to collect her, making Offred believe he has been an Eye all along, which she had secretly feared. However, Nick tells her that he is a member of Mayday and the van is here to take her to safety. Offred doesn't know if she believes him, and it is not confirmed if Nick is truly a member of Mayday or not. However, in the epilogue, it is mentioned that one Commander Fred Waterford - who may have been Offred's Commander - was arrested and executed by the regime in the early years of Gilead for harboring an infidel in his household, who could've been Nick. This would imply Nick was indeed a rebel and helped Offred escape, though what ultimately became of him is unknown.
In the Hulu series Edit
Nick is portrayed by Max Minghella. He is originally from Michigan and his name before the rise of Gilead was Nick Blaine. He had a brother named Joshua.
Before Gilead Edit
Nick is unemployed and sitting in a career center. He is talking to a counsellor, Andrew Pryce, when he gets in a fight with another customer. Nick is thrown out, Pryce follows him and invites him out for coffee. Nick tells him about his brother and the hard times they've had, and Pryce tells him about a religious group he is part of called the Sons of Jacob that wants to "clean up" the country.
After the takeover Edit
Nick, presumably in his new job, is driving Mr. Pryce, Mr. Waterford and Commander Guthrie. The three men are discussing women's roles and The Ceremony in the new society they want to build. When asked for his opinion, Nick replies that the three men's idea of the new society was right and would be best for everyone. This also evidently indicates the first time Nick meets Mr. Waterford. 
Though not shown on-screen, Nick is later hired by - or assigned to - the Waterfords as their janitor and driver. This is probably also the period when he, on his (presumably frequent) tours with Commander Fred to the brothel "Jezebel's", and the kitchen maid Beth engage in a friendly sexual relationship, to which she later refers as the "old times" . It is quite possible that Beth introduces him into the Mayday network (though not confirmed, see Beth/Notes). Nick also keeps working for Pryce, now Commander of the Eyes, by evaluating reports of snitchers, in particular about Commander Guthrie.
Before the main storyline Edit
While watching Guthrie being led by two guards into an office building of the Eyes, Nick reports to a pleased Commander Pryce that Guthrie has been sleeping with his last two Handmaids and is skimming from the transportation budget, according to his aide and his Martha. Pryce then remembers Nick to "be chiefly reporting on the activities of his own Commander" and regrets the loss in Waterfords' household. He concludes "We are going to clean up Gilead, son".
In the presentEdit
While driving Offred home from the Putnams, Nick is asked by Offred about Ofglen's arrest, and he advises her to remember she "can't change" this and "being tough or brave" won't work because "everybody breaks", and she should "tell them everything". They then see a black van parked in the drive, to which Nick apologizes he "couldn't stop them". Inside the house, Offred is interrogated and electrotortured by an Eye and Aunt Lydia about Ofglen. Nick later brings her ice for her injuries. He confesses he wished he'd just driven away with her .
Serena Joy calls Offred outside to tell her she's asked Nick to try to impregnate Offred. She suspects the Commander is infertile. Serena Joy secretly brings Offred to Nick's apartment, where she waits as they have sex. They finish, and Serena Joy tells Offred to lie down in her room, after saying a prayer to encourage her to become pregnant.
After an emotional conversation with Fred, Offred gets sick and goes to throw up in the kitchen sink, where she runs into Nick. She asks him if he's an Eye, and he tells her yes. He apologizes for earlier, and explains that he couldn't say no to Serena Joy. Offred later sneaks into Nick's apartment to share a passionate night with him.
After the dinner with the Mexican delegation, Offred visits Nick, fiercely blaming herself for keeping quiet about the brutal truth of her fate. Nick tries to calm her down and calls her Offred. She reveals to him her real name June.
Nick drives Fred and Offred to Jezebel's. In the brothel, Nick visits Beth in the kitchen to trade illicit goods, such as prescription drugs and pregnancy tests that are used by the customers or working girls, for bottles of alcoholic beverages and additional contraband. Nick formally comments the trade with "the Eyes thank you for your service". He refuses her advances. 
Serena Joy returns from her mothers house the next morning. As Nick brings her bags through the kitchen, Offred notices Nick's aloofness and asks if she can see him later. When he says he doesn't want to see her anymore because it's too dangerous, she demands a reason stating that if she indeed is murdered for her actions, at least someone will remember her. She mentions that she knows nothing about him and he tells her that his full name is Nick Blaine and that he is from Michigan, but that is all. Offred leaves the room upset. 
After driving Fred and Offred into Jezebel's a second time, Nick visits Beth in the kitchen again. Nick is curious if she "asked around at the bar" that "nobody's planning anything stupid", and Beth replies that the girls in here are not "in the shape for a rebellion". She confirms that Offred's identity has not been discovered, warning Nick about the dangerous consequences of "poking around, asking questions", or "being sweet" on a handmaid. 
In Night, Nick finds a distressed Offred in the kitchen with a scar on her forehead. When he asks if she was okay, Offred reveals that Serena Joy found out about Jezebel's and that she is pregnant. Nick is shocked but still convinces Offred that the pregnancy is not a terrible thing. They shared a brief tender moment before interrupted by Serena Joy.
In the last scene of Season 1, Offred is in her room waiting for someone to come and punish her for her defiance. Nick comes in, tells Offred to "just go with it" and insists that she trust him. Two armed guards come in and lead Offred away, much to the surprise and horror of Serena Joy and Fred. Offred walks off willingly, unsure if this is her end or a brand new beginning.