I like my music's volume high enough not to hear you.
Seungsoo is a music spirit created by Harmonia. Although he is immortal, his physical age is 18. He currently resides in camp and can be found in the Nymph Sanctuary.
Seungsoo is quite a cheerful but lonely persona. He enjoys the comfort the presence of others brings him, yet can never manage to draw the presence of others towards him. Therefore he confides in his music. He enjoys composing songs or dance routines, or basically just listening to music. To relieve stress, he often does singing exercises.
Seungsoo is normally a peaceful and rational soul who rarely holds an ounce of anger within him. He strives to make others happy, even if he can't do so for himself. To him, others' happiness is his happiness, and that's what he's stuck by for so long.
Music has certainly changed over the years as do humans and everything else in the world. Harmonia saw this in every detail of the progressing humanity and found fascination in it. Subconsciously, she had festered this fascination into a body of its own. To when the idea occurred in Harmonia's head is unknown, but the goddess decided to create a spirit of music simply to admire the changing world with her. It was awfully boring up in Olympus anyway, having someone to talk to would do the trick.
So the goddess of peace created Seungsoo.
Seungsoo stood by the goddess side simply for the purpose of conversating about music and occasionally playing and singing some of his own creations. Seungsoo didn't mind but wanted to have some sort of independence so he kindly requested that he'd be able to venture out into the world. Harmonia was reluctant to let him go, but she did so anyway.
The spirit did as he asked, wandering around the world and exploring different cultures through their music, before he finally came to New York to rest in Camp Half-Blood. One day he might go out to explore once again, but for now he was satisifed with the Nymph Sanctuary.