This is very similar to the low-poly piece I did of Onew/Lee Jinki a few weeks ago, and the subject is an artist from the same band (Kibum Key). A friend half-commissioned this from me.
One thing I really love doing in these low-poly pieces is playing around with gradients. Value is one of my favorite tools in all my designs, and I can really do a lot with it here. It creates a sense of space, and draws the eye into the focal point.