Angels Of Death
In this idea, Zack is the one who kills Rachel's parents. He happened to be passing by and saw them abusing Rachel. So he followed them home and did something about it. Unfortunately for him, Rachel saw the whole thing. Suddenly Zack has a tiny child following him around everywhere. After several attempts to get rid of her (not kill or injure, just lose her) he finally gives up and accepts that he has a blonde shadow now.
Eventually they gain a sibling-like bond, with Zack having basically raised Rachel from that point on. She becomes the brains to his brawn and helps keep them from being caught by the police. Rachel prefers to stick to the shadows. Making traps or leading their victims to Zack. It started as a way for Zack to keep Rachel safe when she insisted on helping him with his killing, but it turned into her own preference for killing from there.
When Gray comes to recruit Zack to be an Angel, he doesn't know about Rachel. Zack accepts the deal and takes Rachel with him. They like this new arrangement. Both have a place to stay and don't have to worry about things like their next meal or shelter. They even have entertainment and Zack can kill whoever he wants when they are on his floor. If Rachel wants or needs anything, Zack will ask Gray for it. Gray thinks it's for Zack and gets it for him.
They live like this for at least a few months, maybe a couple years. Then one day the Angels are having a meeting. Rachel shows up in the middle of it to tell Zack that he needs to change his bandages. The others are shocked when Zack and Rachel explain that they're siblings, and that Rachel has been living with Zack the whole time. After they recover, Gray offers Rachel a place in the Angels officially.
You can end the story here, or continue as you see fit. This is just as far as I came up with. Obviously set before Canon, so you can choose to go through canon if you want, although changed to fit this AU, or just wrap it up here. It's all up to you, should you choose to write this idea.
Pretty old idea of mine but I still like the concept of it. The age gap is too big for me to ship Zack and Rachel romantically (plus she's like 14) but I do see them having a sibling dynamic and want more of that. This idea was actually based off some fanart I saw with kid Zack and baby Rachel. That sparked this idea where Zack finds himself as Rachel's caretaker.
