Jack is five years old. He was born in Room and raised in Room. It comes as quite a shock when he finds out that there is more to the world than Room. His Ma was kidnapped by Old Nick, the man who visits Room every night, and she wants to get out. Ma uses Jack to concoct a plan to escape: he pretends to be dead and escapes when Old Nick tries to bury him.
After that, Jack's world is different: it's the world the rest of us live in, but it's a world he has never seen. He has to get used to hospitals, playgrounds, shopping malls, and apartments, things most people take for granted. But for Jack, it's all new. He grows a lot in the course of the first few weeks out of Room, and he comes to realize that Room wasn't his home; it's just a place where something happened. And he's ready to move on.
Jack isn't your typical narrator. To start with, he's only five years old. He's super curious and really insightful, busting out with the kind of philosophical logic only kids have—like when he says, "If I was made of cake I'd eat myself before somebody else could" (1.271). We don't even know what that means, but it sounds genius… and it makes us want cake. A winning combination for any notable quote.
Ma has raised him well. That's maybe the only benefit of living in Room: it's a controlled environment that allows Ma to give Jack 100 percent of her love and attention. And she does. She raises him to be a hero, calling him everything from Prince JackerJack (2.145) to Samson and Superman and everything in between. He wonders why he's so wonderful, and she says, "That's just the way you popped out" (2.433). Maybe that's true, but he's been raised in a way that makes him even more wonderful—and totally brave.
Bravery is important, because Ma eventually puts a lot of pressure on Jack when she asks him to get them both out of Room. Jack really is her only hope, the Obi Wan Kenobi to her Princess Leia, because she has reason to believe that Old Nick will abandon them and they will starve. She tells him, "I'm your mother. […] That means sometimes I have to choose for both of us" (3.287). Whether she realizes it or not, she's putting him through a crazy guilt trip. The last thing Jack wants is to disappoint his mother. She's the only person he has.
Even though Jack is scared, he pretends to be dead (yikes) gets rolled up in a rug (double yikes) and is taken by Old Nick to be buried (yikes times infinity). And even though he's never been in the outside world (and he's five for goodness's sake) he escapes a moving pickup truck and gets help, breaking his mother free from the prison she's been in for seven years. He's a real hero, for sure.
Later in the novel, Jack cuts off his hair and is kind of surprised to discover: "I still have my strong" (5.644). Jack may have a Samson complex with his hair, but he realizes that he's still the same even after he cuts it off. His strength and bravery don't come from his hair: they come from within.
He may be a hero, but he's still a child, and has to deal with the kind of crazy trauma that would send any average adult into therapy. As a result, Jack gets kind of obsessive at times, giving his life order when it has none. His life inside Room has been so structured that he continues to need the same kind of structure outside Room. He counts his teeth often. When he accidentally wakes Old Nick up with Jeep, he says, "I count my teeth five times, I get twenty every time but I still have to do it again" (1.635). It's a way for him to ensure that everything stays the same. He counts his teeth right to left and left to right, just to make sure everything is the same when it seems like everything outside his mouth is changing on a daily basis.
Dr. Clay, the doctor who helps Jack and Ma acclimate to society, tries to nip this OCD behavior in the bud. Dr. Clay: "A little boy I know, he counts the same things over and over again when he feels nervous, he can't stop" (4.823). He doesn't want Jack becoming Marc Summers.
But Jack's a kid. He's attached to small things, like the number five: "I love five the best of every number, I have five fingers each hand and the same of toes and so does Ma. […] Nine is my worst favorite number" (1.117). So even after the talk with Dr. Clay, Jack says, "Ma lets me revolve in the door five times" (4.859). Maybe it's a good thing that Jack spends some time away from Ma. She can't enable everything he does if she wants him to make it in the real world.
After Ma ODs on meds, Jack lives with Grandma. It's hard, because he doesn't understand that Grandma is family he can trust. Plus, she doesn't act like Ma does. She does things differently, like not bathing with Jack the way Ma does.
Over the course of the week at Grandma's, Jack gets hurt a bit, he heals, and he develops a conscience. He feels bad when he keeps six toys instead of five, and he feels worse when Grandma gives him two coins to give to a homeless person and he keeps one. Ma might have taught him well, but her teachings stay with him even while she's gone. Knowing that Ma is still with him gives Jack the confidence to adapt.
By adapting, he learns that he can take control of his situations in certain ways. Yes, he's a kid, but he still has a say in how things work. This leads to an interesting role reversal when Ma gets out of the Clinic and returns home. Jack wants to go back to Room to see it one last time, but Ma is scared to go. Jack tells her, "I'm choosing for both of us" (5.1034). Ma had said the same thing to Jack earlier in the novel, and now Jack turns it around on her. We'll give her credit: she realizes Jack means well, and she takes him back to see Room.
By the end of the book, Jack is still five, but he's changed a lot from his fifth birthday. For one thing, the world is a much bigger place than he realized, and it's growing for him every day. As Jack observes, "I thought all the weird things happened yesterday but there's lots more today" (3.534).
Jack also realizes that he can be his own person, separate from Ma and separate from what others think of him. The outside world views him as a strange object of stunted growth, which Jack realizes when he reads the article about the Bonsai boy. He asserts, "I'm not a tree, I'm a boy" (4.959). He's not just a boy. And he's not just his mother's son. He's a grandson. A nephew. He's just Jack. (But not this one.)