Minutt For Minutt

His character developement is extremely underrated, because he isn't a main character and it happened in the background (and was after s3 totally ignored).

He joined the Krew as a thief who didn't care about anyone. He lied to them, told them that his new gained airbending abilities changed him, but by the next chance he started to steal again and run away. The Krew took him in (Bolin immediately adopted him, Korra protected him, he and Jinora had the "connection") but there was no guilt in him to leave it all behind.

His change started when he was saved from the Dai Li. He really felt sorry and apologized to Mako and Bolin.

The next, and probably biggest changing point for his life was Jinora. They soon built a friendship (and on the long way more) and he gave everything to safe her from the kidnappers. Same when the Dai Li agent captured her. It was the first time we've seen him caring deeply for someone else and wanting to help them (when we ignore the one weak airbender who he spared in the training battles). The peak of his change was - obvious - when the Red Lotus attacked and he was willing to sacrify his life for the other airbenders (Jinora?).