Another word for "understated sexuality"?
You can't throw a bunch of adjectives together and expect English to cough up some magic word. It might happen, but then again, it might not.
If you want to focus on her beauty, use
beautiful, attractive, appealing, charming
If you want to focus on her appearance, you could try
classy, elegant, sophisticated... or even resplendent (depending on the case)
For confidence, try
poised, self-assured
Modesty, in some senses, goes against confidence, but you can use
self-effacing, unpretentious
For understated sexuality in particular, I'm partial to
elegant
(of people or their behaviour) attractive and showing a good sense of style
or, in extreme cases
sensuous
Attractive or gratifying physically, especially sexually:
[Oxford]
PS - In my humble opinion as a reader, I like writers to describe girls with as many specific adjectives as possible, rather than be too laconic about it. Even your description in the post would mean more to a girl than any single word could.
How about winsome:
adj.
Attractive or appealing in a fresh, innocent way
I like chic:
adjective (chicer, chicest)
Elegantly and stylishly fashionable:
The connotation of stylish welcomes a deeper appreciation than most people offer to the elegant woman the OP describes:
1856, as a noun, "style, artistic skill,"
from French chic, 19c. in "stylishness" sense, originally "subtlety" (16c.), which is of unknown origin, perhaps [Klein] related to German Schick "tact, skill,"
from Middle Low German schikken "arrange appropriately," or
Middle High German schicken "to arrange, set in order;"...Etymonline.com
Winsome is a good old English word for that:
adjective 1. sweetly or innocently charming; winning; engaging: a winsome smile.
an elegant demure and a winsome smile describes many a lass in English books.
Someone fitting exactly that description (yeah, she’s currently reading over my shoulder) seems to find a way to fascinate me every day, so it might just be that all those qualities add up to make her “fascinating.”