Sirius is beautiful;
Magic and looks and mind bred through hundreds of years to this one result. Perfect bone structure, perfect skin, perfect, aristocratic profile, and you can almost believe that the blood that runs in his veins is really blue. With his black hair and piercing gray eyes he looks every inch the haughty, spoilt prince he was supposed to be.
Sirius is brilliant;
He is quick as lightning, and only needs to hear or see something once to succeed in doing it. He doesn't study and doesn't listen, but his marks are always the highest, and you find it frustrating. He grasps in an instant what it takes others hours or even days, and in a minute what others don't get at all. He can solve any problem you give him in the time it takes you to blink.
You know the only thing Sirius can't solve is himself.
Sirius has a streak of cruelty the size of Hogwarts;
He is mean to those he is indifferent to, and horrible to those he hates. He uses people like it is his right, and maybe he thinks it is. It wouldn't be the only thing left from his family. He lashes out, and his tongue cuts like broken glass, and the shards are left long after. You think that in some cases, the shards are left forever.
Sirius doesn't think before he acts, and he is almost never sorry. Sirius is rash, Sirius is reckless and impulsive.
Sirius has a capacity for love like no one you have ever seen;
He cares about almost no one, but when he does, he is fierce and loyal, he will die to protect them. Sometimes, you think that Sirius would cross the sea and back if that's what it would take to make those he cares about smile.
Sirius looks like he has everything, but you know that in fact, Sirius lacks the most important things. Sirius grew up thinking approval was love and gifts were attention, and therefore has a screwed notion of what friendship is. You think that, as the years go by, you're doing pretty well in fixing that.
Sirius isn't completely stable;
He has violence and anger bubbling under the surface, and you are afraid that someday he would break and it will all get out. Sirius has hatred, another thing his family left him (although they probably didn't think it would turn against them), and it blinds him to a lot of the world. Sirius is dangerous, as a result of his childhood and upbringing, but you are not sure whether he is more dangerous to others or to himself. You don't know what scares you more.
Sirius is as imperfect as you can get; but if he weren't, he wouldn't be Sirius, so you're all right with that.