For the character to work best only one side of the model should be used, this is to keep the same identical geometry either side of the model, so each side reacts in the same way if/when rigged. Below we can see that once i was happy with a completed arm and horn, as part of the wanted design, i split away half of the body, that was only in the scene for reference. I then mirrored the wanted part of the model to then combine them together correctly merging the central vertexes.

Here I discussed with lewis wether to add mouth geometry or not. We decided that if we were happy with the movment of the mouth in the soon to be toon shaded texture, wether were going to use it or not. Below is are the blend shapes that i made as an experiment. The mouth moved well but lewis was not happy with how it conflicted with toon shader, so the mouth was taken out, and is not to be used.