The Dauntless training simulation stimulates fear in the brain. Four has experienced this over and over. He has been reduced to the child in himself, reduced to Tobias, stripped down to the son of Marcus with that same cruelty flowing in his veins.
Caleb's creation, handed over with worried eyes but steady fingers, stimulates happiness. "Deep brain stimulation used to be used to treat depression," he started to go off the first time, when he was still chasing the solution, but then paused when he realized who he was talking to.
Four isn't divergent, just aware. He doesn't have an Erudite bone in his body or the slightest interest in how this serum works, only that it does.
And it does.
"Tobias." Tris laughs softly in his ear, leaning upward on her toes. She is still small and slight compared to him, despite her war-developed musculature, as she presses her warmth against him.
He leans down and kisses her, committing every second to memory: the sound of her gentle laugh, the resolved desire in her eyes, the responsive planes and curves under his hands. In here with her, he is Tobias because she treasures his name like a secret between them. This is the real part of himself, the part only Tris is allowed to see.
He pretends he doesn't notice that none of this is real. He pretends it is real and kisses her deeply, feels her arms wind around his neck and pull him close. He doesn't crowd, knowing why he was in her fear landscape. He simply holds her for as long as she allows it.
He pulls away for needed breath and runs his hands over the back of her neck, combing through her hair. "I love you, Tris," he murmurs into her mouth.
She smiles and murmurs back, "Love you too."
Christina takes in the dark circles under his eyes (he's not sleeping) and the weary line of his body (he's pushing too hard, but he can't stop because that would mean stopping long enough to think), and her mouth forms a hard line. "You went back in the fear simulation again, didn't you?" she demands, hands sliding to her hips.
Four almost winces at the image, the memory of this friend chatting with Tris and being with her while alive, but he doesn’t because he’s had the belt on his back and has seen the dark pits of his father’s eyes. He has four fears. Grief isn't one of them.
Christina used to belong to Candor. He never had a Candor bone in his body any more than he had an Erudite one.
"Yes," he tells her.
Caleb grieves as hard as Tobias.
"You would have let her die," Four reminds him, voice hard. "Back in the Erudite compound."
Caleb laughs harshly and without humor. "You know what? Save your judgment. The only one who had the right to judge me is dead."
He doesn't know where the instinct comes from that shoves Caleb hard against the wall and holds him up by his shirt. Except he does, because he dreads the monster inside of himself as much as he fears falling from the rooftop or closet doors slamming shut and close and dark around him.
"She's dead," Four tells him in a low, dark voice that lights up the first signs of fear in Caleb's eyes. "It's your fault."
Guilt shows naked on his face before Caleb closes his eyes and breathes evenly, walls shutting away emotion. He opens his eyes. "I know."
Somewhere in the shared pain, they figure out how to tolerate each other. Somewhere in the grief, Caleb asks about the fear simulations Tobias visits because even if he's shooting Tris every time, even if it kills him, he sees her, and he needs that so, so much.
"I could do that," Caleb says. He does not know he'll regret it. "The simulations work on the brain, you know, not just fear."
Not just fear, but also… happiness.
For a few stolen hours, they can slip a needle in their necks and see her because she makes them happy. Caleb uses it to cope and bring himself to some state of peace and resolution. He takes the lessons from his pain like the good Abnegation he was once raised to be and learns how to move on.
It's been a year since Tris died and the city is empty without her. It's been a year and Tobias hasn't moved on.
"You know this isn't real," Tris says quietly as she traces one finger lightly across his collarbone. Her gaze wanders down, sending a flood of heat through his body. She runs her hand lower and follows the line of flame across his ribcage.
They've tangled together on the bed, their bed because this isn't real and there's nothing stopping his mind from creating the peaceful life together they'd wanted.
He splays his fingers across the back of her hand and presses her against him.
Her eyes flit upward to look into his.
"Don't remind me," he says.
Tris is Tris though, even here, especially here, and she doesn't let him get away with anything. She turns her merciless gaze on him as readily as he’d turned his own on her when she’d been beaten and would have done anything to hurt her attackers in turn. "I never took you for a coward, Tobias."
He recoils, then sets his jaw and turns away.
She doesn't let go of him, and he holds onto her hand, onto this moment as long as he can before the fading happiness makes the room fade around him just as certainly.
The loss of her never gets easier.
Christina's making a disgusted noise, tongue slapping against the back of her teeth as she evaluates the contents of his kitchen. There’s less food than rationing requires. There's more alcohol than grief should require.
"I don't remember giving you a key." Four crosses his arms and glares at her.
She shoots him a singularly unimpressed glare and drops a bag of vegetables on his counter. "My best friend is dead, Four. You don't get to follow her into the grave."
He stares at her. He sets his jaw.
"Just don't even start," she cuts him off before he opens his mouth. "Don’t give me the ‘if you wanted to be bothered by people like me, you would have joined Candor’ line, because the amount of care I give this moment is none."
He doesn’t quite recognize this girl glaring at him, and wonders what happened to the Christina he remembers. But then again, the war will do that to you. It'll make you cling tighter to the little things you have left.
He sits next to Caleb at the edge of a rooftop, legs hanging over the edge as if he isn't afraid of heights, and passes Caleb the bottle.
It's Tris's brother, just like Christina is Tris's friend—just like there's a needle waiting for him below in the old Dauntless compound.
"It's still your fault," he says lightly but somehow it doesn't mean the same thing it did before. There's no hatred between them.
Caleb looks at him with sorrow in his eyes because this isn’t healthy and he knows it, and he hands Four the serum anyway.
"Yeah. I know."