He did stay. For years, as both a friendly thunderbird and occassionally as a sort of older brother. Hogwarts passed in the blink of an eye, and he made sure Rose enjoyed the majority of her time.
It made him happy, watching out for her. Day by day, week by week.
She married at some point, and he blessed her wedding. She looked beautiful, and he kept a picture of her in her wedding dress with him to remember.
Then it was over. The older he gets, the more it seems like life blinks away in a heartbeat. It was here that his decision was made.
Standing just in front of the fresh grave of his counterpart, Rose Potter Black, he made his decision.
"I want to help more than just the magical people. Maybe I have too much to bear, but I'll take all I can and try my best." he declared. His wordsdrifted away in the wind, and he felt the stone on his ring burn hot. A cloak he often left undisturbed and unseen felt like ice around his shoulders, and a wand that didn't quite belong in his hands felt like thorns stabbing his hand.
"In death, there is relief. I want to relieve people before they have to die."
Harry's head felt like it was ready to split open. Before, he had thousands of strands, a web of magic, drifting through the world. Now he had millions. At least, it felt like millions compared to thousands. In all reality, it was an infinite growing to an even greater infinite.
With a solemn nod to the grave of Rose Potter, Harry vanished.
Line Break
The next thing Harry saw after his farewell to a long time friend was a war zone. The change of scenery was something he hadn't experienced in over a century, but he still processed the scene in an instant.
It was a wrecked desert. Pieces of scattered cars and weapons were littered throughout the place like shrapnel, and only one figure seemed to be moving at all, a man in a metal suit.
The man collapsed, metal helmet falling off of his head as he laid down, squinting up at the sky.
"You alright man?" Harry asked curiously, walking up to the man. He had dark hair and eyes, and he looked rather ragged.
"Shrapnel. Headed into my heart. Magnet stopping it from killing me." the man answered with several gasps.
"Why the hell would you use a magnet instead of just getting rid of the shrapnel?" Harry asked incredulously.
"No surgery instruments." the man argued. "Probably would have died."
Harry cocked an eyebrow. "Like you aren't dying now? Whatever man. I'll fix it."
Harry used a little magic and popped the metal suit open.
"Wow. You really should have just removed the shrapnel." he said, before getting to work.
With a combination of basic medicine, magic, and the fact that a piece of metal was the biggest obstacle in his way, Harry fixed the man's chest.
Harry snorted. Really, he just vanished the shard of metal and cast an episkey, and that pretty much fixed everything.
"Can you do something smarter next time someone puts some metal in you? It's better to just removeit, rather than use a fucking magnet to keep it from moving around." Harry griped.
"How did you do that?" the man asked. "That was a bad wound... and I feel fine?"
"Lay down idiot!" Harry ordered. "Just because you feel fine doesn't mean you are."
Engines and the sound of a helicopter approaching could be heard.
"Are you good with the Americans?" Harry asked conversationally.
"Yea. Better than I was with these guys." the man replied weakly.
"What do you want me to do about this stupid armor thing?" Harry asked.
"I think I'm done with arms dealing. I'd rather it get buried."
Harry nodded and vanished the remains of the metal suit.
"What's your name?" the man demanded weakly.
"Harry Potter. You?"
"Tony Stark."