Central character of Andersen's fairytale, a girl... boy from beneath the sea.
He was born to the sea-king, master of the merfolk, and knew the love of his father, his grandmother, and five doting sisters. He lived a pleasant, simple life, uninterested in the bizarre tools that fell to the seabed from the world above. To him, there was only his beautiful garden, filled with flowers that could only be found in the realm of the merfolk. The sole exception to this was but a marble statue that he happened upon one day. That statue, a simple one of a young man, would go on to become the centre of his garden, and would ignite in him a boundless curiosity for the world of men. He hounded his wise old grandmother with questions, and was astounded by her tales of fragrant flowers, green trees and singing fish that were called 'birds'. Unable to rebuke his excitement, his grandmother informed him that he and his sisters would be allowed to visit the shore above upon their fifteenth birthday.
And so, the days did pass, and each of the little havmand's siblings did reach the top of the sea, bringing back new tales for him to drown himself in. Hearing of bathing in the moonlight, seeing the golden sunset, hearing the human children, feeling the winter chill and simply being in the upper world only incensed him further. After being forced to wait and wait and wait, until his sisters had grown indifferent to the top of the sea, his fifteenth finally came, and he was allowed to swim up.
When he reached the surface, what he saw was a ship, large and loud. The joyous music and the jubilant lights made him daring, and he moved to get an even closer, until soon he was looking in through the cabin windows. It was then that the boy caught sight of a prince, and so a fairytale of love and misery would begin.
Unable to take his eyes from the beautiful prince, the merman remained until a storm arrived, laying waste to the ship and scattering its crew to the seas. Realising that the humans of the world above could not live in the seas, and that if allowed to sink, then he would not be able to be with the prince, for the prince would be dead. And the prince could not be allowed to die. And so the prince's life was saved, he was dragged upon the shores and spared the fate of drowning. He would never know that the merboy had saved his life.
Returning to his home, the boy learned the nature of mortality, and that he would be able to gain an immortal soul, like that of a human, if he could fall in love with a human and be loved in return. Now determined and without wavering, he made a pact to receive a human body in exchange for his voice, and took his first step onto the sandy shore, knowing that, were the prince to marry another, his body would become mere foam on the sea the very next morning.
... The story from here on is known to everyone. The prince fell for a woman he mistook for the one who saved him, the speechless prince's heart was broken and, refusing to take the life of the one he loved in order to return to the seas, he tossed his body to the waves and became foam, only to be reborn as a child of the air.
As a child of the air, he would go on to bring happiness to children the world over, growing closer and closer to that day when he would be allowed to attain the soul that he had so longed for. Or so it would have gone, had he not encountered a young boy by the name of Hans Christian Andersen. A boy who had known the nature of suffering and heartbreak from an early age, and thus already believed that 'the only eternal happiness that can be attained by humans is death'. Thus, even if he were to hear the spirit's tale, even if he were to be begged and appealed to for years, he could not be brought joy. And so it was that The Little Merman, without bearing a grudge or ever finding fulfillment, found his spirit scattered and left as dust.
It would not be until decades later that a much older Andersen, in memory of his love for the man Edvard Collin, composed the tale Den lillie havfrue, a familiar tale with a female protagonist. Curiously, the story would originally have featured a far more bitter ending, one where the title character died and dissolved meaninglessly - a fitting end for Andersen's writings. And yet, for reasons unknown, Andersen revised this ending, created the far more hopeful conclusion in the final work and claimed it had been his intention all along. Perhaps it is this fact that forms the basis of Caster's wish - 'to give that boy eternal happiness'. If there was even the slightest spark of hope in Andersen's heart, then Caster has devoted himself to that entirely.