This will only include the original 25 characters, since I've only watched 3 seasons.
Sierra was meant to be a comical relief and to give Cody some attention. Personally, I thought she was nothing but annoying. I couldn't stand her and whenever she was on screen I just wanted her to get off. She did nothing except telling things about Chris and Cody, and helping her team win challenges. She did become a much better character in the last 5 episodes, but she was so unbearable for me during more than 20 episodes, so she's ranked last here.
Katie and Sadie are like one. Sadie is lower than Katie due to being on more episodes of TDI, but not evolving anything. Not at all. K&S were also meant to be comical relieves, but I only found them funny a couple of times. I did kinda like them in the Aftermaths though, where Sadie actually did evolve. Due to Mr. Coconut evolving more in one episode than she did in 3, she's this low on my list.
Katie is the other half of Katie and Sadie. Katie didn't do much in TDI, and she didn't really do much in the Aftermaths either. She made me laugh once or twice, but there were other much more funnier characters out there. She placed higher than Sadie due to the fact that Sadie only was noticed twice by me during her time spent alone - when Katie left, and when Katie returned to leave together with her.