You simplified the forms very well. Your lines tend to get a bit thick in certain areas. Make sure to keep your lines flowing and loose.

Fun drawings! Try challenging yourself to include more straight lines, as even slight curves can be simplified into straights. That can add a new element to counterbalance the more complex curving edges you already integrated.

Hi Jin :) This looks beautiful. My advice would be when deciding where to place your values, consider the direction that the light is being projected from, and try to group lighter values on the parts of the subject that are closer to that light source and the darker values on the parts of the subject that are further from the light source. With that in mind, since the light source is coming in from his left, I would lighten the values that define his cheeks on (his) right side of his face, and push the darker values on the left side of his face. You've done a great job with this portrait, can't wait to see more!