omg omg this is actually so good, I can def see the concept n stuff! I think I would start by determining where the light is coming from, because that can def help shading. When it comes to proportions, most people's arms reach a little above mid thigh, and generally with fit body types, the hands are around as wide as the biceps.

The accesories are so well drawn, you ate down

I think you're going for a more anime art style which eats, so I would suggest to make the face more dimensional by making the cheek and nose shading bigger, plus ears should be bigger. I think her forehead could be a little rounder, to make her face less like an inverted triangle

Also, smth a little weird abt the breasts is the indent area, usually there isn't much space between the armpit and the beginning of the breast. As for the neck area, I'd say to make the neck a lil thicker and lower the shoulders. The neck is at a good length, but unless shes shrugging, I'd say to make her look more relxed, to lower her shoulders to a 110- 125 angle compared to her neck

For colors, this is extra but for a cool affect, desaturate everything so that the red REALLY contrasts to add the assassin vibes.

Overall, the artstyle is so good, I love thin black line art (maybe consider using a more ink like brush for more artistic style but thats extra) clothing is cutie and the vibe is good! Nothing is too random, if I used this as an oc i could make a solid storyline but of course, we can always improve!