You need a lot of work on your shading. You're trying to mimic the style of the original sprite, apparently, but you have to realize it's not at all compatible with the spritesheet you're working with - not all sprite templates are created equal, and you can't handle them the same way from one to the other.
Specifically, your character looks rather dull. This is largely because you're lacking any semblance of highlights at all, and very minimal shadows.
Also, he's either going bald, has the biggest forehead ever (and then some), or you need to move your hair down quite a bit. Remember that with almost any competent sprite sheet for the XP, and the corresponding tilesets, you're looking slightly down on the characters, so you shouldn't be able to see as much forehead as even looking straight on. But, even if you had that completely angleless view, your character still has a receding hairline. Again, don't follow the exact proportions of the original sprite, just the basic features. And on that note, IMHO, his hair comes down on his right side and I don't see that in your character.
But, all that aside, it's a strong strong start, and I admire your ambitious choice of the HK for your first sprite. It's definitely a lot of fun and by far my favorite, but there's plenty of spriters here that would consider it a more advanced template. And, my boyfriend would cream himself to hear someone's trying to interpret Tactics, he loves that shit.
Good start, keep going. (`o`)b