First, characterization doesn't need to occur in chronological order. You can have a character who then turns out to be more than he appears, and development then revolves around how he continues to overcome a past or adopt yet another change in his life. Second, in order to have drama you need flawed characters. It's like the fundamental different between Star Trek TNG and Star Trek DS9. Everyone was smiles and happy times much of the time in TNG, and so the show had to feed off more of the episodic scifi stories. In DS9 you had characters that actually produced conflict with each other. As for Yukine, he was a lost teenager who didn't truly understand his own situation and acted the part fairly well. You might not find this cool, heroic, clever, or upstanding, but you should at least be able to empathize with his position. At first he thinks of ways to use his condition to a material advantage as he doesn't like being without money, doesn't like wearing hand-me-downs, and doesn't like being forced around. He also doesn't consider the impact his own actions have on others. Again, like a teenager. Eventually he comes to understand that none of that actually matters anymore and comes to accept what he is now.
That's growth.