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, assigned to the household of Commander Fred. Offred finds Nick attractive; he apparently returns her feelings. He approaches her in the house, alone, one evening after she slips out of bed and speaks with her, which is forbidden. Nick also 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. After 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. This is evidently how Nick first meets Mr. Waterford. 
Though not shown on-screen, Nick is later hired by 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).
Before the main storyline Edit
While watching Guthrie being led by two guards into an office building of the Eyes, Pryce reveals that Commander Guthrie has repeatedly been violating Gileadean law, and deeply regrets the loss in Waterfords' household. He orders Nick to spy on Mr Waterford and report all his future activities. Pryce concludes "We are going to clean up Gilead, son".
In the presentEdit
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 moment with him.
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, Offred reveals to a shocked Nick that she is pregnant, and they share a brief tender moment.
In the last scene of Season 1, Offred is in her room waiting for someone to come and punish her. Nick comes in and tells Offred to "Just go with it" and insists that she trust him. Two armed guards come in and lead Offred away, 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.