Ace-Zach. They met when they were kids.
Zach is from the poorer parts of town, plagued by violence and gang activity. He's had a rough childhood. His mother left when he was very young and to this day he has no clue what happened to her. He was left behind with his alcoholic father, who spent more time passed out on the couch in front of the TV than paying any attention to his kids, and his older brother. His brother had dropped out of high school and become involved in gang activity pretty early on. He and his friends often physically abused Zach -- tying him to objects and depriving him of water and food, hitting him, holding fires and cigarettes too close to his face. He was still very young -- I believe nine or ten -- when he sat on the curb to the street he lived on as the setting sun cast its orange glow on the playground across the street. His bruised face was buried in his hands. He contemplated running away, but where could he go?
Despite his living conditions being from a "bad" part of town, it was near a gated group of houses where the wealthier families lived. So on this day, as young Ace walked home from the playground, he saw Zach.
Ace asked him what was wrong, and why his face was black and blue. He tried to understand, but couldn't really, being a young child (a year younger than Zach). Zach couldn't really explain. He didn't really know what was happening either.
So Ace brought Zach home to his family. His parents were very shocked to find a battered/bruised kid alongside their own kid (who, in my mind is wearing a bowtie and dressed like a stereotypical nerd). They patch him up and ask what happened, and he explains. Being adults, they are able to catch onto what exactly happens in Zach's household. They contact authorities.
Zach's father is sent to rehab. His brother is sent to juvie. Zach is able to be put into a foster home far away, but Ace's parents decided to take him in. The boys had seemed to grow a liking to one another. "I've always wanted a brother!" Zach remembered Ace telling him. In a way, he's always wanted a real brother too.
So life went on. Zach kept his scars, and studied hard (though when he started, he was so behind that he fell a grade behind -- into the same grade as Ace). He wasn't the smartest kid ever, but Ace definitely was, and loved to teach his brother/best friend. Ace never had to worry about getting picked on, as most of the "tough" kids were terrified of Zach.
Zach absolutely believes that Ace was his guardian angel. Had they not met, where would he even be this day?
Though Ace is headed off to a big university soon, to study his rockets. He's a talented musician, mathematician, scientist... none of which Zach is. He's no good at anything. He's a little worried about Ace moving away, but he hopes they'll find some way to stick together.