Relatability. Readers need to be able to relate to the characters in a story in order to care about them. This doesn't mean that the characters have to be perfect, but they should have some qualities that readers can identify with.
Empathy. Readers should be able to feel empathy for the characters in a story, even if they don't agree with their choices. This means that the author has to do a good job of showing the characters' inner thoughts and feelings.
Conflict. Great characters are often in conflict with themselves, with other characters, or with the world around them. This conflict can provide tension and drama to the story, and it can also help to develop the characters' personalities.
Motivation. Every character should have a clear motivation for their actions. This motivation can be anything from a desire for revenge to a need for love or acceptance. The more complex the motivation, the more interesting the character will be.
Flaws. Great characters are not perfect. They have flaws that make them more relatable and interesting. These flaws can be anything from a bad temper to a gambling problem. The more flaws a character has, the more real and believable they will seem.
Archetypes. Characters often fall into certain archetypes, such as the hero, the villain, the mentor, and the love interest. These archetypes can help to give characters a clear role in the story and to make them more recognizable to readers.
Originality. While archetypes can be useful, it's important to create original characters who don't simply conform to stereotypes. The most memorable characters are the ones who are unique and unexpected.
Voice. Every character should have a unique voice that distinguishes them from the other characters in the story. This voice can be expressed through the character's dialogue, thoughts, and actions. The more distinct the character's voice, the more memorable they will be.