A good character arc, and that alone. Why just that? Because it encompasses everything else, and a character is nothing if there isn't some kind of story they can tell.
A good character arc requires that a character have some sort of flaw that they would then proceed to overcome by the end of the story.
It also requires that the character have some want or goal, a motivation, and for that motivation to play a role throughout the entire story.
Together these two things, a flaw and a motivation, make the character relatable. We all have flaws that we want to overcome, and we all have a goal that we want to reach. Baking those things into the DNA of the character instantly makes the reader empathize with them.
This relatability is also responsible for a character being memorable. The more personal the struggle of a character through their arc, the more memorable they become to a viewer.
A good arc also almost guarantees the story the character is a part of as a whole will be at least somewhat enjoyable.
Notice the ideas here have nothing to do with physical appearance, design, personality, or social identity. That's because these are the things that are external to a good character. No matter their appearance, personality, race, gender, or identity, all good characters have the building blocks of a good character arc to stand on.