Perhaps in response to his dull small town life, our seventeen-year-old narrator, Cullen Witter, has formed a veneer of sarcasm and utter pessimism:
Being seventeen and bored in a small town, I like to pretend sometimes that I'm a pessimist. This is the way it is and nothing can sway me from that. Life sucks most of the time. Everything is bulls***. High school sucks. You go to school, work for fifty years, then you die. (1.13)
A large part of Cullen's pessimism comes from feeling boxed in by life. Unlike his brother, Gabriel, who is perfectly happy sitting at home and reading a book, Cullen feels bored and confined in his small town. He feels like this little, lifeless place is sucking him in, and when Oslo dies and Gabriel goes missing, things just get worse for him. All of a sudden, he's mostly alone and has to spend his days constantly thinking about the same thing—the depressing, terrifying, awful disappearance of his favorite person in the world.
In fairness, this would get anyone down in the dumps, not just a jaded teen.
Cullen may be a pessimist, but that doesn't mean that he has a cold rock where his heart should be. In fact, when he's dealing with the people that he loves, he is a downright sap. When Gabriel mentions that Cullen should kill him off in his zombie book for a cool twist, Cullen thinks about how that could never be a possibility:
I didn't tell him then, but I had no intention of ever letting him die in any book. (3.43)
He may not say his feelings out loud a lot, but Cullen has very strong, protective feelings for his younger brother. He would basically do anything in the world to make sure that his brother doesn't get hurt in any way, which is why it destroys him inside when Gabriel goes missing and there's nothing he can do to bring him home. Outwardly, it may seem that Cullen is taking everything very well, but he's a raw, angry, sad mess inside.
Despite his pessimism, Cullen is also somewhat of a romantic—especially when it comes to Ada Taylor. She's the girl that he's pined after for all his life, and he's thrilled when she starts showing him attention:
At some point over that next week I had decided that even if Ada Taylor was showing me attention and affection purely because she felt sorry for me, I was still madly in love with her. (11.10)
The problem is that Cullen's romanticism isn't actually based on Ada Taylor—it's based on his fantasies of her. He's built her up so much in his mind that when he actually gets a shot with her, he can't make a real relationship work. She's more of this beautiful mirage than an actual human being to him. Pro tip: Relationships with mirages rarely work out.
Since Cullen kind of hates his life, it makes sense that he'd escape a lot in his mind. In fact, he's constantly coming up with all these book titles and imagining scenarios in which he gets to fight off zombies and become the hero of the day. As he says:
When one is sitting in the passenger seat of his best friend's car, and an overly enthusiastic hillbilly is ranting in the backseat about being snubbed by a cheerleader at lunch, his mind begins to wander and think about zombies. (1.48)
Cullen's mind is constantly wandering to zombies because he wants to escape from the real world—and when Gabriel goes missing, these zombie fantasies only become more frequent. After all, Cullen feels incredibly helpless in the face of his brother's disappearance, and his fantasies are the one place where he can save Gabriel and bring him home for good.
The moral of the story is this: When it comes to Cullen, he might put on a cynical show, but deep down inside, he's always hoping against hope. And in the end, his steadfastness is rewarded: His life returns to the boredom he's always known and loved to loathe.