When they're fifteen, Ben dies.
It takes Klaus less than a day to find out the whole thing was orchestrated by their father. Reginald tells Luther it was his fault, to keep his Number One in line and drive home the message that he had to be perfect or else his family perished. He critiques Allison's attempts to resuscitate Ben, telling her everything she did wrong in her desperation. Vanya isn't even told until a day later when she timidly asks at dinner where Ben was, unaware of the loss thanks to her siblings being forbidden from grieving 'too loudly'. He makes sure Diego feels guilty and weak for not being there, too ill to go on the mission.
Perhaps if you were stronger, Number Two, and been well enough to help your teammates, you may have been able to stop the sniper before they shot Number Six.
At the time Diego hadn't questioned why Reginald wanted him to drink a foul tasting glass of cloudy white liquid. Their father was always making them try things and would record the findings, telling them they had done well, even when they were writhing in pain from whatever experiment he had conducted on them.
And he tells Klaus that maybe now he would have sufficient motivation to improve on his powers; he had someone specific to find now.
Summon him, Number Four. I do not want to hear of strangers, I want you to summon Number Six.
So Klaus does. He summons Ben after hours of crying silently into his pillow. He summons Ben alone in his room and sobs because Ben can't shed tears as a ghost. After crying for Ben, Klaus listens.
Oh he listens.
Contrary to what most believe about Klaus, he was no idiot. It isn't hard to piece together really, in the end. Reginald ran on the belief that he had total psychological control over the children and at no point thought they might doubt his words, that two of his possessions might gather together in a bedroom at night whispering to the spirit of their departed third member of the even numbers club. Arrogance, nothing but arrogance, led to Reginald losing three of his Umbrella Academy in the span of twenty-four hours: two more than he had planned.
Ben only just manages to get Klaus to stop Diego from going down to Reginald's room and slitting his throat then and there. He tells them to leave, escape, and bring their siblings with them. They try, oh boy do they try.
Klaus sneaks into Allison's room to try and explain it all to her, but she just slaps him and tells him she's not in the mood for his cruel, stupid games while he's high. In a room further down, Diego goes to wake up Vanya but she whispers 'What if Five comes back and we're gone? He won't know where we are'. Diego spitefully tells her he didn't want her to come anyway, he only asked for Klaus and Ben's sake, hiding his anguish at her answer behind bitterness.
Even after seeing Klaus' slap mark and Diego's tight set jaw, Ben still begs them to ask Luther too. Luther deserved a choice for once in his life.
It ends with Luther gasping from a cut on his leg, Allison bleeding through her fingers as she shields her damaged mouth from Klaus' kicks, and the Even Numbers Club vaulting out a two storey window and onto the ground below: Diego's shoulder dislocating, and Klaus' ankle snapping. They steal one of Reginald's cars and nearly run Luther over when he runs out to try and stop them, swerving around him at the last moment and flipping him the bird as they zip by. Diego slams the pedals down hard, speeding them through the city so fast Ben can't track the scenery going by the window, while Klaus leans over to steer, and they attract the attention of the police for miles until Diego orders Klaus to drive them off the bridge and into the river below.
Arrogance is what lost Reginald three of his agents. Trust was what freed two of them. With absolute trust in Diego, as Diego had him when he came to him telling him he thinks their father killed Ben, Klaus yanks the wheel around and sends their car smashing past the bollards. They fly.
They fly until they sink.
Reginald tells the media that Numbers Two, Four and Six were tragically killed by cowardly snipers on the Umbrella Academy's recent mission. He has his car pulled up from the river by private contractors paid to keep their mouths shut, and near shakes with anger when the classic hasn't a single young man's body within it.
Luther and Allison stand alone as the last of the Umbrella Academy for a year. A year of hearing nothing from their brothers. A year of cultivating hatred for them. For accusing their father of such a heinous, unthinkable crime. For running away as deserters. For hurting them when Luther and Allison tried to stop them. Reginald hammers it into their brains so hard even Vanya begins to believe it all.
A year later, on the anniversary of Ben's death, a knife with a note attached comes crashing through the window as what remains of the Umbrella Academy eat breakfast with their father, and buries itself in Reginald's chest.
With trembling hands Vanya reads the note while Luther carries his father off to the infirmary, their android caretaker in tow, and barely blinks when Allison snatches it off her to read too.