A critical part of the Divergent series has to do with Tris and her friends dealing with their greatest fears. This is a part of Dauntless initiation. Most notably, each initiate must go through his or her fear landscape (a simulation in which they must face their greatest fears) in the final part of initiation. In Allegiant, many of the characters fears are alluded to or mentioned, but they are never dealt with specifically. Below is a list of each major character from the story along with a one of their greatest fears. Following each fear is a possible way to get past the fear if they were to encounter said fear in their fear landscape.
Tobias: His greatest fear is losing Tris, without being able to do anything about it. Of course, he faces this fear at the end of the story, and the only way to surpass it is by continuing to live, without giving in to the hopelessness and loneliness he feels without her
Tris: A fear of Tris's that is seemingly overlooking in Allegiant is having her parents die for nothing, or to have her parents not be proud of Tris's actions. Ever since reading her mother's journal, Tris's bond with her mother has grown stronger than ever. She thinks often of what her mother taught her about sacrifice, and it always worries her that she will not make her mother proud. If she were to experience this in her fear landscape, she might watch herself die selfishly, and then have to look her mother in the eye, shamefully.
Caleb and Cara: these two characters, both Erudite, possess a similar fear -- their rationality and intelligence failing them. If they were faced with a problem that seemed simple but turned out to be entirely unsolvable, they would feel defeated and inferior.
Christina: One of her greatest fears is losing her family, who remain in the city throughout the story. In her fear landscape, she would have to watch her little sister and her mother die brutal deaths, while she could do nothing to stop it.
Peter: A key plot line in the story is Peter's quest to reset his memory so that he will no longer be a cowardly, self-serving person that likes to hurt others. In his fear landscape, he might be unable to escape these character traits that he is so disgusted by. He might take the memory serum, only to find that it has no effect on him. He would have to cope with the fact that these traits are so ingrained in him that he can never escape them.
David: David fears two things, powerlessness and losing Tris's mother. He is in a position of power, being the leader of the Bureau. In his fear landscape, he might find himself as a GD, forced to do maintenance in the compound for eternity. Also, his love for Tris's mother is touched upon in the story, but the reader has no idea how painful it must have been for David to watch from the compound as Tris's mother chose to assimilate completely into the city experiment, leaving him behind. If he had to relive this loss of Tris's mother, he would be in misery.
Obviously, these portions of each character's fear landscape are only hypothetical, never actually having a place in the novel. Much can be learned about the characters, though, by imagining what it is they fear more than anything else. Fear landscapes shed light on a person's strengths and weaknesses, as well as their moral values and soft spots.