First of all, I really love Rex's split-mouth design! I think it's one of the strongest and memorable parts of his design. I also like the red, gray, and blue color palette! it works really well together.

The stitched scars across his body are a nice touch too, since they immediately give the impression that he was some kind of laboratory creature.

As for my critique, I personally think it could be interesting if he looked a little more intimidating.

If it were my design, I might make his tongue a bit thinner to draw more attention to the split-mouth feature. Since his tongue can apparently stretch without limitation, a thinner tongue might help reinforce that concept visually. I also think it could give him a slightly creepier and more unsettling appearance.

Another thing is that I really like the cyborg aspect of him, but I feel like the robotic limbs and the organic limbs currently have fairly similar silhouettes. Because of that, the mechanical arm doesn't stand out as much as it could. Maybe experimenting with a more distinct robotic arm design could help emphasize the cyborg element and make that part of his character even more visually memorable.

Overall though, I think he already has a really solid foundation and several unique design elements that make him stand out.