You generally want all your characters to be “sympathetic,” including your villains.
Problems make a character sympathetic. IE Everyone loves an underdog
Relatability (IE everyman qualities) makes a character sympathetic because we see ourselves in them.
Expertness (IE superman qualities) makes a character sympathetic because we aspire to be like them.
Niceness makes characters sympathetic. But remember, even though villains are not nice people (typically), they can still be sympathetic if they have other things going for them.
Note: The Hero’s journey can be thought of as the journey from the everyman to the superman.
Note: Remember that a “super power” needn’t be “super” in the comic book sense. IE in Oscar Wilde plays the characters are super humanly skilled socially, in LotR Samwise is super humanly loyal.
Note: Characters must typically be more consistent in novels than people are in real life. If you do something out of character, you must foreshadow it somehow.