He was just there to fill in, but I thought even TJ and Derrick would end up continuing a relationship, I know I even thought about that before the whole working together as medics to save lucian alliance members lives.
My bet around the end of the episode of the final series, TJ dies because of her uncurable disease, does something to save the crew. Which will give Young something to think about, and why it would of been best to stay on that planet. Blame Rush, because of his desire to find the background radiation of the universe. But there has to be some sacrifices.