How do they usually dress? If he's trying to blend in or just lounging about, then he wears commoner clothes in shades of black, gray, and dark browns. While travelling and on a hunt, he dons darkened leather and fur armors with a cloak and cowl. He doesn't like to wear the actual Dark Brotherhood armor as he thinks it draws too much attention despite the fact that dressing head-to-toe in black is probably just as bad.
What do they wear to sleep? While sleeping in the sanctuary or at an inn, he switches back and forth between preferring with a tunic or without, but he always wears pants, at least. If he's sleeping in the wilderness, then he keeps all his armor on and wraps himself up in his cloak like a cocoon.
Do they wear jewelry? Before being taken away from his parents' slaughter, he stole a gold diamond necklace from his mother's jewelry box and kept it hidden beneath his shirt. Once reaching the Dark Brotherhood and undergoing training, Moarte had it enchanted with Fortify Archery. He is constantly going back-and-forth with himself on if he should continue keeping the thing or destroy it once and for all because of the ties to his past.
Is there anything about their appearance they wish they could change? Moarte is the type of person that couldn't care less about his physical appearance. If he had to stop and think for a moment, then he might wish he was a tad bit taller, but it's not very important and he doesn't lose sleep over it.
How would they look as the opposite sex? A little shorter, very faint curves, small bust, longer hair, face would be less of a square shape and fuller - nothing drastically different, I believe. If Moarte were female, most likely he'd care a bit more about his appearance.
What do they smell like? Why (do they wear the scent or does it occur naturally)? For the most part, he literally smells of death -- a mixture of just turned earth, stale and fresh blood, muggy musk and smoke, and a faint hint of sweat. It's a very odd combination that he doesn't notice most of the time. After he's had a bath, though, he has a general damp, clean smell mixed with whatever odd herbal or floral mixture he decided to use.
Do they have an accurate mental picture of their appearance (how they see themselves versus how the world sees them)? I believe so. As I've stated previously, Moarte really does not care how he presents himself or how he looks, for the most part. He sees himself for what he really is -- just another young adult wandering in the world, falling into a whatever lifestyle he deems fit. Others outside of the Dark Brotherhood see him as either odd looking, trouble, slightly adorable, or just a normal boy.
Moarte’s features that stick out the most would probably either have to be his physique or his eyes. (In reference to body build, though, I’m mostly comparing it to in-game instead of comparing him to other OCs.)
Physique - He’s rather slim and slender for his age and hardly holds any muscle or body fat on his frame. Most of the people he’s come across, even the wispy looking Altmer and Bosmer, have some more meat on their bones. It’s possible that Moarte’s just a runt or his very poor diet has had an effect on his body, or both. The only small bit of muscle he has on his body is situated on his arms and legs from either years of archery practice or constantly traveling across Skyrim.
Eyes - Even though he has blue eyes, which are incredibly common in Skyrim, there’s still something about them that’s very eye-catching, no pun intended. He practically wears his emotions on his sleeves, so even if he’s trying to cover up how he feels, you can almost always tell by looking at his eyes. They’re also always watery and glassy and have a childish gleam to them that can be a bit eerie when he’s out on an assassin job, if his victim stops to pay attention.