Introduction
This image started out as a lunch break exercise, but I liked where I was going so I decided to take it a bit further. In this article I'll try to reveal most of my workflow for this particular piece, covering all the main steps to reach the final image.
Concept
The main concept was something I'd had in my mind for a while, inspired by the Axolotl salamander (Fig.01).

Fig. 01
I heard of a warrior challenge for Mudbox. I couldn't officially participate since I was focusing on developing my ZBrush skills at the time, but I decided to do it for myself anyway.
I chose to create a female aquatic warrior character riding a giant manta ray. Her gills would be exposed, as in the Axolotl salamander,r and would help to define the female gender of the character by giving a sense of waving hair.
I had actually done some quick sketches for this character before and that's probably the reason why things came out so easily (Fig.02).

Fig. 02
Base Mesh
Since it was supposed to be a ZSketch of one hour, I started a new base mesh from a simple ZSpheres structure. Basically, I used one main ZSphere for the base of the neck, with two ZSpheres to the side for the shoulder area and one going up for the neck. Then I added another ZSphere for the base of the head, followed by one main skull ZSphere and another for the mouth/nose area.
To the main skull ZSphere I attached the gill structures and the eyes, as well as more in the back of the head. This was all done with symmetry turned on (Fig.03).

Fig. 03