I must admit, when it comes to certain characters, I'm a bit skeptical on whether they are Sues or Stues as you call them. Sues are generally those characters that can practically do everything in the universe without even getting up from a sitting position and be drop-dead attractive and get everyone to fall in love with them (from my point of view at least) or could actually be an ugly character that, despite what everyone thinks, would end up being with a character that hated them in the beginning and was drop-dead gorgeous... But there has to be at least a little spice to a character as long as there isn't too much or too little.
For example, I'm using my newly re-established main character from a book I plan to write later on.
I created Sai along with the other characters with a different intention... but later discarded or changed their details because I found that they seemed to have too much activity to them and seemed too important, which I don't want to make it seem although he is a main character within my book.
Anyway, Sai is a half-Japanese American teenage boy that is 14 years old and has unusually colored hair... which is pink. I will explain the reason why that is the case though. He used to be a pale blonde, but he wanted his hair dyed red, but instead of coming out red, the color mixed and now his hair became pink. Sai already believes his reputation has been damaged enough (even though he knows he didn't have much of one established in the first place), so he decides to keep it pink... However, with his green eyes, he does seem remotely like a boy without the eye drawing to his pink hair.
Another part to Sai's story is the fact he was a regular human... at one time... but this will be explained later as to why he is no longer a human per-say. Sai doesn't have a sadstic past, or the death of anyone special within his life, in fact, he has quite the opposite type of life-style. Sai lived in a regular home with his loving mother and father and his twin brother Dai. Unlike Dai though, Sai likes writing and plans to writing a book someday while his brother plans to be an artist (mostly using calligraphy brushes and painting more than anything). The both of them however had to be moved into a private school after Sai unknowingly upset demons and cursed him and his brother because of it. The private school is suited to fit these types of situations though, but it's rarely talked about because the school is located within a mountain. The school is expensive though and Sai and his brother both have to take up jobs within the school to pay it off. So you could say despite the curse and the fact it's a "private school" you could say he's a tad bit over the normal line.
Onto what I commented about the fact of him no longer being human... Well the case with that is now, because he upset a Nekomata demon, he gained cat ears and a cat tail. So, you could call him a demon. But don't go off saying he's an over-acheiving Gary-stue yet, because unlike most people that gain demon powers and abilities... he has none at all... Because a demon cursed him, he only has the outer appearance as a demon, but has none of the abilities that they have gained.
Honestly, I thought before that I could make him the deadly sin of Envy... but then I soon realized that it would be too much for his character because he was already "technically" a demon as you call it... so I just decided to add it as an aspect to his personality... to where Sai has the tendensy to get jealous at certain things... but not go over-board with it like he would if he would've been if he was the literal sin "Envy."
I'm not too certain how Sai's character pans out, but he's mildly interesting without going too overboard from what I feel. But that's just about it... And I feel that at this point... his character could go either way as the story progresses... Hopefully it's for the best.