How do they usually dress?
For a long time she lived under a shadow of an ex-queen and princess and Victoria couldn't have been a better mentor for Soojung. She was used a way more than the mermaid, and the youngest followed her steps and innocently started to treat the woman as if she was the Mother she had lost long ago. And what daughter doesn't put on her mother's clothes?
So Soojung started dressing up like a real princess, a human one would - classy even if casual, never slutty, even if at least half of her closet were made of shorts, short dresses and jeans not as loose as the other girl's.
(She was a mermaid after all, and one with the luck to have stunning legs to show off.)
What do they wear to sleep?
Away from the tight, short clothes and casually uncomfortable clothes, the word for her night was comfort - that when she remembered taking off her clothes before passing out. But well, when her memory worked, old nightgowns were the answer for the perfect clothing for the end of a tiring day. Yes, the ones your grandma would probably use. Soojung used them all before her anyway.
Only if necessary, when she's asked for something important on university or even at work, or at some friend's celebration. Fancy jewelry is not her favorite - at all, but she had been using a bracelet and a necklace she won on Christmas. (Maybe just because she was sure it was something her Father would have gotten her if he had the opportunity.)
Is there anything about their appearance they wish they could change?
Honestly? She wished she hadn't change so much for something that wasn't worth all the world she lost. She wished she was a mermaid again, if she had her family with her, it is. If not, it doesn't matter how she looks really.
How would they look as the opposite sex?
Do you mean if she was a guy? ...Uh. She'd probably have shoulder-lenght hair as her Father, maybe a little of a beard? I can't picture Soojung as a guy, I'm sorry.
I've had better thoughts, and inspiration, and the FC I seemed to have found to be the most similar to Soojung, would be Jung Jin Young; with hair red and slight long, bangs covering his deep eyes, enchanting voice and that tiny smile of his, there is no better.
What do they smell like? Why (do they wear the scent or does it occur naturally)?
Soojung doesn't use fancy perfumes. She tried once - when she first became a human and she was put in big bathtubs and perfumed with all the best and exotic perfumes a royal family in 1830 could find - but it all seemed so not her, it was ridiculous.
Nowdays, she tries to keep it simple. Deodorant (fragrance free), and how she smells depends on the body lotion she's using at the time, but generally, its sweet and salty but not too much. Some have said she smells like the ocean.
Do they have an accurate mental picture of their appearance (how they see themselves versus how the world sees them)?
Her appearance is a soft spot for Soojung. She knows how she looks - she has a mirror, duh - but it doesn't pleases her. She owns the most beautiful pair of legs, given by the Great Lion himself to her, but it is like they aren't there. Or explaining better, she didn't want them to be there and that's why she doesn't care. Her hair though, complimented by many, is what she likes most - maybe because she remembers how soft and red her mom's hair was when she was still a toddler.