As readers we like our characters to be realistic. Authors need to add flaws and virtues to their characters in order to engage the reader with the story. We remember the characters who come alive best. Those characters have flaws. They have virtues. They have clear goals too. Character development should include a few flaws for the hero as well as a few virtues for the villain.
That said there are a few things to note. It’s best not to imbue your character with too many virtues and few flaws. I hated the character of Pollyanna in the best-selling 1913 novel by Eleanor H. Porter, even though I never actually read the book, only saw the movie. She was too good to be true, but tastes change I suppose. I didn’t like Dorothy from the Wizard of Oz either for the same reason. Neither character rang true to me. I suspect the reason was they had too few flaws. Yes, Pollyanna had a mischievous aspect, but it wasn’t enough to overcome her nauseating optimism. Not for me in any case, which means you may feel differently. Dorothy didn’t seem to have many flaws at all. She was brave and a natural leader, and she only exhibited doubt once or twice in the entire book. I would have preferred her to be a bit more daunted. Both characters were a little too much on the sanctimonious side for my taste.
Harry Potter, on the other hand, was brave, courageous, but he also was a little diffident and not above a little bit of cheating. Ron Weasley was a coward, who stepped up when needed. Hermione was a know-it-all and a bit bossy, but she was also loyal and honest. The three young wizards are more realistically drawn than Dorothy or Pollyanna. The three from the Harry Potter books are more interesting because they are a nice mixture of flaws and virtues. Dorothy and Pollyanna are not as interesting to me because the mix of good and bad is so off as to be nearly nonexistent. Now that could be because their stories were more cautionary tales about being good, than pure entertainment, although L. Frank Baum claimed in the foreword to The Wonderful Wizard of Oz that the book was written as a modern fairy tale for children to enjoy.
Modern readers want characters they can relate to. They want characters who have problems. Characters who have flaws to overcome much as the readers do themselves. This is why character development is so important. You want to create characters that readers will relate to.
A good well-rounded character is a mix of flaws and strengths. Write your characters as a mix of good and evil and you will have a believable, well-rounded character. A well-rounded character is one that comes alive on the page. A character that is not such a mix is a paper doll. There’s not much they can do. So make your character good, but mix in a bit of evil and watch the character come alive.