The moment Diego figured it out it wasn't that much of a surprise. In fact, he already knew, after all, he and Vanya were close growing up. They had started their own rock band at one point, Diego foolishly thought that he and Vanya would always stay that way. It was futile though; Vanya could be blamed to an extent. She had left everyone behind to escape their abusive father and then wrote her tell-all book. In hindsight he couldn't really blame her, she was hurt and if the book helped her heal he supposed he couldn't be angry. It was better than destroying the world at any rate.

So, since they were close in their original timeline Diego was privy to things that the other was not. For example, he knew that Vanya had a hopeless crush on Five before even she did. The girl was always numb, rarely giving in to her emotions. Yet when anyone mentioned his brother Vanya's face lit up. It wasn't obvious unless you looked but Vanya was always so much more tolerant with five than everyone else. Sure, she was a pushover but for five she wasn't. She challenged him, not because she wanted to but because it was what five needed. So, she broke her character to challenge him but then would also make him his favourite sandwiches when he was upset, an act she did for no one else.

He knew that Vanya had fallen and fallen hard when Five disappeared. She would absolutely refuse to admit he was gone. She even argued with their father, which had never happened before. Reginald simply glared at the girl and ordered her to her room. She had then gone on a hunger strike, she wanted Reginald to admit there was a chance that five would come back. It was stupid and futile, but Diego guessed he could understand it. She was they only that spoke about him, well other than Ben. He only did that in private though, avoiding dads wrath.

It had been a week since Vanya had started her hunger strike when Diego had had enough. He made her some food and stormed into her room. She looked up, startled but he faltered. Her eyes looked sunken like she hadn't slept in days. Her hands were shaking from hunger and if it wasn't for the fact she had the good sense to stay hydrated Diego had no doubt she would be in the infirmary. He walked towards her gently and placed the sandwich in front of her.

"You need to eat, five wouldn't want you to starve on his account." He murmured. Vanya laughed; her eyes pained.
"I won't eat until dad at least tries looking for him" Diego ran his hands through his hair.
"He's gone Van… you don't need to die to prove your point. We all agree with you but Five.. he will only be found when he wants to." She didn't reply and he sighed, he got up and turned to leave when he heard sniffling. Vanya had started crying, he sighed and turned to her.
"I just miss him" she admitted. The boy frowned; he had never seen Vanya look so distressed before. She was usually pretty unemotional.
"I miss him… I think… I think I- "Diego placed a hand on top of hers, understanding in his eyes.
"I know," he said. He then pulled her into a hug as she sobbed.

It was then that Diego knew that whatever had happened between Vanya and Five was a bit stronger than being siblings. Yet he still didn't know what they were.

Well, he didn't until they time travelled back into their thirteen-year-old bodies. It was when they reset the timeline that he noticed Vanya wasn't the only one who had a certain adoration for a sibling. Five himself seemed to act differently around Vanya. He would always find reasons to touch her, whether it was a quick handhold or a hug he always found reasons to do so. More often than not it was he who initiated physical contact with Vanya which was weird. Ever since the apocalypse, five hated any form of physical contact. He would flinch when anyone tried hugging him, any clap on the back would be met with resistance but here Five was actually wanting to touch another human being.

At first, he let it go, maybe Vanya just knew how to get through to five or maybe Five was trying to let her know that she wasn't alone. It wasn't until one night the penny dropped for Diego. He had gotten up for a glass of water when he heard sounds from Fives room. Curious, he tried listening through the door. He heard sobbing, a loud, heart wrenching sob. He knew it wasn't Vanya as he could hear her trying to reassure the person crying. As such the only logical conclusion was that it was five crying. That in itself was a rarity, never had he seen the unmovable boy cry. He could hear Vanya reassuring him.
"It's okay Five, I'm here," she said. Diego smiled slightly, of course; she was there. It was five, she was always with Five. Five sobbed even louder
"I… I killed you… I hurt you" he choked, Diego froze, when did he hurt Vanya. The girl in question shushed him lovingly.
"It was a dream five, I'm right here."
"Don't leave" he pleaded.
"Never" she replied. Diego took a step back, feeling like he was intruding on an intimate moment. He grinned nether the less, it seemed that Five wasn't the arsehole he pretended to be.