The key is to humanize them (in a general sense rather than a specific species!) - and by that I mean humans have flaws, troubles and weaknesses that are relatable to the reader. They don't have to be ones that the reader has personally experienced - just that they can understand how unpleasant that might be to live with.
To illustrate what I mean:
He is adopted to a noble family where he is raised to be a "perfect gentleman". He has the class, talent, grace, and manners of a noble. Not only does he have unusual features, he could also be classified as attractive.
Many dislike the "noble" classes because they see them as priviliged, living an easy life where everything is handed to them by right of birth rather than by hard work. So make that "class, talent, grace, and manners" be something that his family drilled into him mercilessly as a child, maybe he wasn't allowed to eat dinner until he could correctly identify all the cutlery, punished if his courtly bow wasn't perfect, forced to learn endless facts and family trees of his own and other noble families. Now the majority of your readers won't have grown up in a noble family - but they will likely have had at least some experiences of striving to meet standards they felt were impossible, demanding parents pushing them to do better, or being told they can't go and play until they finish they're homework and get it right etc. And readers can extrapolate from that.
And by showing that his supposed "perfection" wasn't because he was special - but because he worked really hard at it helps avoid a classic complaint about this sort of character. If you can carry him having to work hard to achieve mastery over most things that makes him "perfect" all the better.
His "uniqueness" made him a loner.
Instead of him being a loner (someone who chooses to be alone) instead he's lonely. He has no "real" friends or leisure time because who he spends his time with and how he spends it are dictated by the family to the family's ends. Again most people will have had experiences where external life factors prevented them from doing as much of what they wanted, and most people will have at least some experience with feeling lonely at somepoint, so once again the reader can extrapolate from their own experiences.
You get the picture.
Spoiler alert, the reason for his appearance and why he's effortlessly attractive? He's a siren. White-haired and heterochromia are common in their kind. And yes, it's also common for their kind to water bend.
If he's getting bonus attractiveness and kick-ass magical powers out of being a siren there needs to be balance. It doesn't have to completely cancel out the benefits it brings him, but it should be significant enough to matter. Perhaps sirens are particularly persecuted (Evil tempters/temptresses that come in and steal and corrupt people!!), or those that can manipulate water, or even those who can do magic in general are distrusted seen as evil (Burn the witch!!).
A common complaint about over-powered characters, Mary Sues and the like is when they don't have that balance to them.
Take Batman - he is pretty awesome - he's a genius, he's extremely wealthy, and kicks ass all while looking good. But it comes at a price, his parents were murdered, and while living as Bruce Wayne he has to constantly present an extremely fake persona to protect his life as Batman. That and of course doing his thing fighting crime and various villains drastically limits his ability to have "real" relationships, both platonic and romantic.
I'm sure many people would still make the trade - I mean you get to be Batman, but the important part is that it's still got the cost, and it's still a cost most readers would consider significant.
