Here is some concept art for Zech.

Since Gene does have a demon side to him, I thought of the idea of Zech having a bit of a demonic influence in him as well. only instead of a demon form like Gene would have, I thought of the idea of him having a kind of darkling form (I like to think of it like the devil trigger from Devil May Cry).

So, this is some concept art for that form.

This is the second sketch I did for the concept. While overall there are some better details, I think it lost some of the aggressiveness of the first one.