Rex gets so much development in the Clone Wars TV show, and I felt like ahsoka didn't get good until season 3, and capped out for the most part after season 6.
I just feel bad for rex. He woke up every day to see his brothers die in battle. Also his close friends like fives, echo, hardcase, and others all died. He was then practically left alone. He rarely saw cody throughout the end of the war.