She will tell people she's an elf, but strictly speaking is mostly something else. She has inherited Sebastian's silver hair and green eyes, but often dyes her hair in less memorable colours. Her figure is small and slim.
Another trait inherited from him is a relationship with the Lady of Luck, leading to odd streaks of coincidences.
Shii appears to be competent with magic and potions of various sorts, though it has been stated that she has trouble with fire spells in particular. Potions and magic items are a basis for her to live on, but she's also a thief and fraud, financing her occasionally expensive lifestyle.
A compulsion for pranks and causing trouble led to some serious problems with a sorcerers' guild. She has the bad habit of contradicting herself even within the same conversation, which makes her not quite as good a liar and actor as she likes to think.
She eats way more than her size and thinness would suggest.
Shii has said her father Dolen died because he was mistaken for Sebastian - presumably he was killed because of one of Sebastian's crimes or someone he offended.
It remains to be seen how she will get along with Midara.
When she first met Sebastian and Midara, she was about 150 years old.
At the time of Molto Vivace she travels with Jason.