This was really beautiful. I don't think you've used purple prose so much as old fairy-tale language, which is always more formal in its delivery.
That said, would you be terribly offended if I offered some minor corrections?
And so quiet
As the child never needed care, it was left in a house on the outskirts of town. The townspeople sent one person every day to see if the child was still there, though no one made any real attempt to help or harm it.
At the age of five, Duo Maxwell ran away. The townspeople needn’t have worried about his return; he had no intention of ever going back.
Five may seem too young an age for a child to be out on its own, but the case of Duo Maxwell is a special one indeed.
It was not his fault that he was born with the innate ability to steal Hearts - he hadn't even been conscious of doing it the first couple of times it happened; he had been a baby! - but as he had gotten a bit older, he had realized what he was doing. Gaining control over his abilities, he of course had stopped doing it.
It was not Duo's fault that his companion still had a job to do.
So Duo had seen no choice but to flee.
When Duo set out from his hometown, his companion had made only one demand
His companion had no problems with that; Duo soon found that it was because no one took him seriously when he said it. He got over that quickly though.
Seemingly out on his own at such a young age, there weren't many options for Duo. The solution however was rather obvious to him: he was already set to steal Hearts for his living, so why not just extend his practice? Duo became a thief; not just of Hearts, but of money and clothing and food when he wanted it (not that he really needed the last of those things, but he had found that his existence was a lot easier when not afflicted with hunger).
the first place they looked for the culprit was in the slums, which was where Duo often hid.
Solo too had seen death many times, though he did not know of Duo's companion; Duo was very careful not to tell him.
His companion advised him against going back again and again, but Duo found that for the first time in his life he was happy, and he wasn't ready to give that - or Solo - up yet.
He threw himself into his work, for now he truly knew the truth: he was Death, and Death walked beside him; all he would know was Death, and Death was all he would have.
and he withdrew from the world: he ran away again.
He climbed carefully and quietly through the trees towards the motion and noise he could sense just ahead until - suddenly and without warning - he reached the outer branches of a tree at the edge of a clearing.
Having not seen anyone in quite some time, Duo was quite shocked
And your danger has not yet passed; Death may still take you.
Duo turned to look at the boy again, who looked curiously between Duo and Death, content to watch and not speak. "He can not keep my heart," Duo said firmly. "Where is he keeping it; I will steal it back."
"But he only has one Heart, and if that is my Heart, than where is his?" He addressed the boy with the blue eyes once again. "Who are you, mortal? What happened to your Heart, and why have you taken mine?"
They passed no one and nothing on the way.
He was from a small village far away, peaceful and quite settled in the woods.
But Heero, even deep under her spell, had resisted, so she sent him into exile to chop trees forever
But Duo's Heart had broken the spell on him, and now Heero was going to back to his village to stop the Witch and free his people.