Spectacular use of light and shadow in my opinion, creates a contrast that highlights each of the two characters but also intricate details of the drawing and meaningful elements. Ok lines, perhaps I'd appreciate more firmness, but I'll attribute that to style rather than quality; vivid colors, notable specially in conjuction with the aforementioned light and shadow and a balanced ammount of details, think it wouldn't take too much to make it perhaps polluted or lose a bit of clarity but I think you managed to not get there. I'd perhaps appreciate a background, think it could fit nicely, but oh well.
Also, is this a fandom reference or an OC? If it's a reference and I knew it I would have liked to talk a little bit about it considering that, but maybe some other time.