One of the greater and more overlooked character improvements seems to be Rick Flagg to me.
Dude was just a grumpy lovesick soldier in first movie, now he is a sarcastic, caring-father figure to team. I loved the look he gives to Harley during rain, all the sarcastic replies to Shark, Peacemaker and Ratcatcher and even Thinker as an enemy.
Lets hope his aligment wont cause him to sacrifice himself to save a bunch of expandable psychos, but comrades at heart.