If you want a character to stick in readers’ heads for a long time after they finish your story, having a fleshed-out personality helps, but it’s not good enough. There’s an enormous difference between A) a character who is shy, tries to be brave around her boyfriend, wants to break free from society’s constrictions but is too lazy to do so, and can’t figure out how to care about strangers and B) the same character, only this time, she always wears a bright pink scarf and loves the study of insects.
Harry Potter has a scar, Katniss has a pin, John Egbert has a green ghost shirt, and Ash Ketchum and Finn the Human both have special hats. An item of clothing or extremely striking physical feature will stay in a reader’s mind far more easily than “defined cheekbones” or “large, blue eyes.” When you give physical description, try to give an unusual visual marker at least to your main character. Not necessarily to set them apart from the other characters or mark them as special within the story, but to set them apart from the hundreds of people your reader sees every day on their way to work or school.
Likes, dislikes, and obsessions make your characters pop off the page like no other character traits can. Sure, your protagonist is arrogant, but wouldn’t your story be more interesting if he were arrogant and liked pizza more than almost anything else? It doesn’t even have to be a plot point. Hobbies and interests make characters more human. Sadly, I’ve read many stories where the characters were only bundles of traits who only formed opinions about other characters and never on which TV shows deserved to be cancelled. Such characters are not fun to read about and probably need to be fleshed out more.