"That Anima of yours...when you're a little bigger...will you give it to me?"
"Hmm? Okay." -Fly & Cooro (Ch53)

When Cooro meets up again with Fly Greena in the last volume of the series, his personality changes completely. Cooro, who smiled all the time and was full of energy, had morphed into a quiet and melancholy young boy. His friends could not understand what had happened or why Cooro was the way he was around Fly. They began to assume that since Cooro grew up with Fly that Cooro was actually a man-made +Anima and he had betrayed their trust in him. Nana quickly turned from him in anger and Cooro asked them to leave, after all, he did owe Fly.

We know that Cooro made the promise to find +Anima for Fly, but what did Fly ask of him in return? Fly was always fascinated by Cooro. He thought Cooro to be the most pure and perfect +Anima: his ability to sense other +Anima and his pure, black wings. When Cooro was younger he approached him to ask him for his wings.

At that time, there was no way that Cooro could have known that he could lose his +Anima. He had always had one, so why should someone be able to take it away from him? But Fly never let Cooro forget his promise and when he eventually found him in Sailand, Cooro followed through. He felt that he couldn't back out on Fly's request, every request was so important to Cooro and he took it to heart.

But what exactly was Fly after when he asked for Cooro's wings? At the time, it sounds like such a strange request. How could you take the wings and give them to yourself? Even though Fly was expert at removing +Anima, he was also experimenting with implanting +Anima in other people. An experiment he was perfecting for himself.

Fly was quite literally obsessed with a strange creature (girl) that the scientists named Blanca. She was compared to Cooro in a way, though she was not a creature of the world. She was somewhat of a spirit, a physical manifestation of the hopes and dreams of the scientists, a concentration of spiritual energy. They experimented on her, trying to inject her with anima, making her a guinea pig of sorts. All the while, Fly wanted nothing more than to Fly with her but only if he had Cooro's pure, black wings.
"Such lovely wings. No other Anima is this beautiful and pure. I will be taking them. You did promise." -Fly (ch 53)
The Royals did not believe her to be a spiritual being, but a fake +Anima, as Fly was known to produce. Then Fly challenged them, asking them if they did not believe in God since they said Blanca was no angel. They stammered and became quiet, only to watch Blanca, fascinated by her. As soon Blanca was exposed to the outside world, she took to the skies, it was then revealed that Fly had implanted Cooro's +Anima in himself and he was finally able to fly with her.

"I never thought much about my +Anima before because I've had it since I was born. That's why I made that promise to Fly and get out to find +Anima." -Cooro (ch53)

The thing about +Anima is that they can never truly leave the human body when forced, and Cooro learned this. When he returned to the field when he was born. He was watching the crows in the field and his friends found him, stunned, because even though he had his +Anima removed and given to Fly, he had black wings. Though, they found out that it was the anima of the crow in the field, and not Cooro's true +Anima. They called him back and he fell to them in tears. He had forgotten was he truly had, these children were his true family. Tears of joy ran down his cheeks, in that moment, the King and royal guards watched as Fly disappeared with Blanca, Cooro's +Anima returned to him when Fly seemed to disappear into nothingness along with Blanca. Fly worked so hard to steal what was Cooro's only to find that it rejected him and in the process, it may have killed him.