I love how expressive she is! You said you were going for cold, composed, mysterious, and you nailed it!! I feel like you can glean those things about her right away from her facial expression and her body language.
The one thing I would advise being mindful of is her pose. At the moment, it seems a little stiff and rigid. That may or may not be intentional- but if it is intentional, feel free to ignore everything that follows this sentence!
I hope you don't mind, but I did a quick sketch to highlight a few things you could work on to make her stance feel a little more natural.
One thing I've noticed watching people is that men (and I'm generalizing here) tend to stand with their shoulders pulled up higher and their legs farther apart. This often communicates confidence, strength, comfort, or even discomfort/dominance depending on how they are holding themselves.
Women tend to be a bit more relaxed in their posture, with their legs closer together and their shoulders more sloped/relaxed. I noticed that on your OC, her legs are farther apart than the norm you usually see on how women stand (again, painting with very broad strokes here), and her shoulders are held high. If you were going for a more boyish/confident type, then that's great! If you wanted her to seem a bit more relaxed, feminine, or natural, I would advise giving her hips and thighs, sloping her shoulders slightly downward, and having her legs be a bit closer together.
One other thing I noticed on your OC was the hands. I think they are slightly smaller than the norm. If you are going for a more childlike character, then small hands is one way to show youth. I made her hands larger on my sketch - to make her look a bit older/more mature. I also looked @ photo references for the hands - and advise you do the same for hands, postures, body language, facial expressions, clothes, everything really! Photo references are a great way to draw inspiration and understand how things work.
I hope you continue with your OC and your art!! It is really quite nice and you did well
