He's a black swan, he can turn himself into a black swan, and he can turn himself into his human form.

Legend says, black swan is has a great power and loyal to whoever meet them and being kind to them.

One day, he's in his black swan form getting hurt and you save him immediately, in that time you still 5 years old. And now, you're 19 and meet him in his human form, but you didn't recognize him in his human form.