Character development. Basically events that change the character of the character in the story.
What exactly is character development?
In fiction writing, character development is the process of building a unique, three-dimensional character with depth, personality, and clear motivations. Character development can also refer to the changes a character undergoes over the course of a story as a result of their actions and experiences.
Is character development necessary in anime?
Characters doesnt need to develop but they need to be in-depth. Of course a show can survive on only one or two good character developments. Usually the only exceptions are episodic comedies. I feel like people still use this criticism way too often.