The day Shiro died Rin made a decision. Fighting never is a solution, so instead of defeating Satan he'll become a doctor to heal everyone who was hurt by him. That'll be his atonement.
Disclaimer: Don't own it.
The day of Father Fujimoto's funeral was gloomy, with dark clouds dominating the sky. It felt like the sky itself was crying tears for the departed soul.
Blue eyes looking at the grave of his beloved father sadly.
If only I'd been a better son and stayed out of trouble.
Father Fujimoto had said to him again and again that fighting never was the solution, but he had never listened. Getting in trouble again and again and look where it had landed him.
In front of his fathers we grave.
He wanted to kick Satan's ass, avenge his dead father. But he also knew that this was not what the father had wanted for him. He could still remember father telling him that revenge never solved anything. Only nurtured dark feelings.
But still he wanted to be of help. And if he could not fight in revenge he would heal. That day Rin made a decision, the most important one he had ever made.
So when Mephisto asked what he wanted to do now, to choose he did not hesitate to answer.
"I want to become a doctor. I'll heal all those who have been hurt"
Mephisto had laughed at him, called him a native fool but still had complied with his wishes. He'll show them all by becoming the best doctor he could be.
I'll keep this a one-shot. If any of you want to complete this or write about this idea go ahead. Just inform me first so that I can read it too.