Character Analysis
Damaged Goods
Tobias experiences identity crisis after identity crisis in Allegiant. We've lost count of how many crises (crisiseses?) he faces in the book. He finds out he's not Divergent. He finds out he's "genetically damaged." He's still having issues with Mom and Dad.
He conquers some of these fears (does that mean he has to change his nickname from "Four" to "Three"?). He gains some more. And he makes some really stupid decisions that end up killing a friend. Oh, and his girlfriend dies, but at least that's near the end of the book.
Let's start with the fear part. Tobias is "afraid of [his] considerable capacity to kill" (23.62), and he's not sure exactly what he wants to do with this insane strength of his. He doesn't want to be a revolutionary like Tris: "I'm not sure I want to offer myself to […] solve humanity's problems." (2.23).
But when he finds out that he's genetically damaged, he gets mad at the society that did this to him. He teams up with Nita's little group of insurgents not to solve humanity's problems but to perform an act of revenge. He doesn't realize that this won't fix anything. He won't become undamaged just by killing a bunch of people.
When Nita's plan goes awry, a bomb blast puts Uriah in a coma, and he eventually dies. Great. Add "guilty conscience" to all of Tobias's other internal problems. He's able to come to terms with this by apologizing to Uriah's family, but just as killing people won't un-damage his genes, apologizing won't bring his friend back. Maybe that loss is the wake-up call Tobias needs to not let his anger control him.
Two Halves of a Whole
One thing that Tobias is sure of is the fact that he loves Tris. How to actually manage this love… that's another issue entirely. He and Tris have the same fight over and over throughout the book: Tobias always asks Tris for her opinion but then completely disregards it.
This really comes back to bite Tobias when Uriah dies. Tris isn't too happy, either, knowing that this death could have been averted if Tobias would have just listened to her. Tobias still can't get it through his thick skull: "I'm wondering what bothers you more, that I made a stupid decision or that I didn't make your decision" (33.98), he says.
(Actually, that's a good question. It's not like Tris can see the future, right?)
Despite being mad at him, Tris loves Tobias, no matter what. She understands that he isn't what society labels him: Dauntless, traitor, Four. He's just himself, and that's fine. She tells him this, shortly before she dies: "It's what you deserve to hear […] That you're whole, that you're worth loving, that you're the best person I've ever known" (41.51).
When Tobias loses Tris, he almost drinks the memory serum to brainwash himself. How can he go on? Tris is the only person who loved him for who he is. Or so he thinks. Thankfully, Christina reminds him that this isn't true. He has friends who love him (at least the ones he hasn't accidentally killed), and he has family, too.
Christina's point is that the easy way out (like taking the memory serum) is no way out at all. Getting through life is hard work. And it's hard work on both ends that leads to reconciliation between Tobias and his mother, Evelyn.
The Parent Trap
Maybe having Evelyn around is the only thing that keeps Tobias going. It's not easy having two evil parents, as Tobias says: "What do you do when your parents are evil? […] Get a new parent" (40.57). In Allegiant, Tobias completely writes off Marcus as the despicable slime he is, and he doesn't feel bad about it, either. We don't blame him.
But life AT (after Tris) would be harder for Tobias if he had to shun both his parents. In the book's final act, Tobias approaches his mother with a deal: drink the memory serum and reset yourself, or give up your position of power and do what's best for the city. Surprisingly, she chooses the second option, putting aside her selfish desire for control and putting the city's—and her son's—best interests first.
Tobias is just has shocked as we are. When Evelyn hugs him, he says, "I can't move, can't speak. She chose me. She chose me" (48.58). In the epilogue, he helps Evelyn, his mother, transition to life outside the city, and he's finally able to mourn the loss of Tris by scattering her ashes over the city. All with a little help from his friends.
Tobias's Timeline