In the attempt to make characters memorable, some authors make them unreal. Sometimes bizarre to the point of being incomprehensible, and sometimes too talented. The complexities of real humans might overwhelm some readers but that is exactly where to start. It is more difficult to make up your character’s personality than to copy it from someone that you know. Combining two similar acquaintances will also work. Basing characters on two or three real people helps to prevent possible trouble if people recognize themselves in your work, and realize that they have been depicted in a less than flattering light.
However, all characters need obvious flaws or areas of incompetence. When people (fictional or real) make mistakes and get things wrong, readers are more likely to remember them. Weaknesses make characters memorable. Flaws are frequently more unique than strengths. Positive traits tend to be similar; such as intelligence, idealism, beauty and physical strength. Another “flaw,” which may not be seen as one, is a trait that prevents a character from fitting into society. This provides an instant conflict if the difference is well-defined. The inability to fit in must create real confrontations that result in suffering to keep the readers wondering how your character will cope with this challenge.
The unlikely strengths are the ones that gain attention. For example, let’s look at the intelligent but uneducated person. They will have gaps in their knowledge, and obvious ones due to a deficit in schooling. The intelligent but uneducated person won’t use fancy words, historical references, or literary allusions. They will be able to describe what they have observed in simple terms and make reasonable guesses as to the cause for events. They’ll avoid making superficial comments without real meaning. Their higher intelligence will allow them to group different distinctions together, or separate ones that other people lump into a single pile.
Finally, memorable characters have an arc and change over time. For example, they may have to deal with a particular flaw of rudeness or arrogance, or overcome a fear and develop a new strength. Even characters whose arc is negative, so they spiral downward rather than grow, are remembered more than characters who do not change at all.