Since my sidekick is my hero's brother I used my hero to design him. I've kept them very similar and changed the the nose mostly. I then started to think about how he will look when the villain posses him. It's quite difficult to show the way he is going to look without colour so the sketches were just to help me figure out some things. His eyes are going to be the same as the villains, not in shape but in colour. He will have veins spider-webbing over his skin coming from his temples and neck and they will spread over his body. he will have gaunt cheeks and chapped lips etc..
Below is some options for my sidekicks hair, I didn't add on all the extras onto his face since I just wanted to figure out his hair. I'm thinking of going for number 5.
Below is just an outline of what my sidekick will look like, When I do the final concept art for him is when I'll be adding all the colour and shading details to him. This shows what his clothes and the colour of them will be. I took his clothing design from the sheet I did for my hero since they are from the same tribe and brothers I wanted them to be wearing similar things.
Below is my sidekicks turnaround from the side you can see his spine sticking out from his back a lot more than a human spine should this is one of the areas that the villain that is possessing him shows through along with his eyes etc..