Shin-Ran- 48. You are born with a small black heart that beats/grows with your soulmate's. It turns red once you've met them.
The Heart beats above where Ran's own heart sits in her chest, small, black, and full of fairytale promise. In all the stories, there's a magical moment when it beats faster and turns from black to red, and you know you've met your true love. And then you get married and are happy forever.
Ran likes the idea of a happily ever after. It's what her parents should have since their Hearts beat together, but reality isn't like stories. Sometimes hearts break even if Hearts beat together. Still, Ran has a secret, tiny hope that they'll get along again like when she was really little. People can't just ignore the other half of themselves. All the stories say so. Even the ones where people try to pretend their Hearts don't beat together end up in love in the end. It just means they have to work for it.
So clearly someday her parents will work things out. They wouldn't still live together otherwise, right?
Ran's Heart is still small and dark right now but it pulses with the heartbeat of someone out there. Do they look at it too and wonder who she is? Do they picture fairytale meetings and happily-ever-afters? She can't imagine what they'd look like. A boy, she thinks, though, someone close to her age. A boy who'd make her smile and hold her hand and make her feel brave even when the world is scary. Ran doesn't know many boys, just the ones at the daycare she stayed at when her parents were busy, but she is going to school soon and there'd be more boys there. People in stories always met their Heart-mate when they were grown up, but Ran doesn't want to wait that long. She wants a friend. She wants a Heart-mate that she knows will make her happy like the stories say and she wants a happily-ever-after with them and her parents and maybe more friends too.
When she's all grown up, she can have a real happily-ever-after and a wedding and everything, but until then she just wants to meet the other person whose heart beats so gently on her skin.
Years and years later, Ran will look back at the memory of her younger self and smile. She meets her Heart-mate and she knows she'll love him from the start. And Shinichi, wide-eyed and young, and older and world-worn, will meet her halfway, loving right back.