They are there for each other at times, and were both involved in Sophia's death because Naomi sold the drugs Cook gave her as a way to pay for her girlfriend, Emily Fitch's, goggles to keep her safe. Cook is shown to respect Naomi, and understands when she stops them from having sex because she likes Emily. Cook also takes the fall for the drug scandal, leaving Naomi out of it. Naomi then puts herself at risk by allowing him to stay with her after he breaks out of jail. Eventually, the cops come after receiving a tip as to where he is. After questioning her she doesn't say a thing, leading to her house being ripped apart, things thrown all over the place by the police.
One of the most bothersome things is that Cook probably never found out about Naomi's cancer, whether she survived or not, (hopefully she did).
They have kissed and at one point were close to having sex, but ultimately they were in love with other people and the love between them was simply platonic, not romantic, although had things been different, there may have been something more between the two. Some view Cook as the rival of Emily for the affections of Naomi, but Cook was in love with Effy, and Naomi was in love with Emily.
They are both seen interacting with others, for example Cook and Emily danced while she was wasted and high and he shared some words of advice on jealousy with her. Naomi and Freddie have shared words on love and getting the girl. Neither of these interactions came close to the Naomi and Cook friendship, however.
Emily, Naomi's girlfriend, has always seemed to have disdain for Cook, while Naomi has painted him in a more positive light to her. For example, in the novel, Naomi sticks up for Cook when Emily goes on about how bad she feels for Effy.
Naomi and Cook seem to spend time together as friends, like when in Thomas's episode in the fourth season, when they are at a pub. In this particular scene, Cook and Thomas get in a fight, Thomas angry at Cook because he had believed that Cook had sold Sophia the drugs. This showed off their friendship again, as Cook keep quiet during the fight, not admitting it until Naomi did, which she did a few minutes after the fight. Also, in this scene, Naomi puts herself at risk by getting in the way of the fight, helping to pull Thomas off of Cook and keep him away. This was just one example of their friendship.
Naomi is sometimes jealous of the way Cook leads a care free life, doing what he wants when the moment calls for it, but he has told her that doing things like that only ends up hurting other people.
In the novel, while Emily is with her family in France, Cook and Naomi bond, both saying that they have grown closer, although Cook seems to have held a bit of disdain for Emily when she returned because Naomi began to care about only her and forget about their bonding a bit.
They are seen to be very similar: fathers that left them, mothers who were not always there (although Naomi's was constantly up or down, while Cook's was always uncaring), they both are have a lot of fear, both have big, loud personalities, and they both seem to not really care about anything, when the truth is they would both die for the people they love.
There are even clips of them in the theme song where you can see their friendship, as seen in the picture above.
The actors that play them, Jack O'Connell and Lily Loveless, seem to be close in real life.
"You're a lot nicer than most people think people think, aren't you Cook?"
"Fuck you right back."