One of the most important things in romance, or in any work of fiction, are presenting characters a reader can relate to.
This means the hero and heroine have to act like real people and yet they also have to be heroic in the readers' eyes. In a romance, they have to be worthy of each other and their happy ending.
There are a great many ways for a writer to learn about the protagonists of their story. Character interviews, which run from a page or two, to a "this-is-your-life" compendium of information-- or not. Some writers may wait until the end of the story to have a full handle on their characters, but in the end, which ever route an author takes, they will know their characters inside out.
Some information is easy, hair colour, family birth order, height, physical descriptions. It is a really good idea to keep track of them, so you can keep them straight. I keep a list as I go along, as well as listing all the secondary characters names and any images or traits I have chosen for them. It is easier to look at a list that track back through the text.
More difficult, yet crucially important, is the character arc for each of the protagonists, where they are at the beginning of the story in relation to the their story goal and in relation to their inner journey and where they are in both of these instances throughout the story, and finally where they are in the end. What they need to do, and how they change to deserve their happy ending that will make the book emotionally satisfying for the reader.
In More than a Mistress, my heroine seems like a very capable business woman who has no need for male assistance, not even when someone seems to be trying to do her harm. Which doesn't quite make sense. Why is she so unwilling to accept help, even though she is very willing to help others? She is certainly attracted to the hero, and not at all like the shy young debutantes he's used to (cough). Each time he helps solve one of the things holding her back from admitting her feelings, another pops up, until finally she admits to the crux of the matter. Even then he has to work very hard to make her see that he is the one person she can trust not to let her down. He has his own arc too, of course.
No matter how you write, pantser or plotter, at some point you probably need to see if you can chart your characters' arc and what it is that is forcing them to make the changes required to achieve a happy ending they may not know they want. In a romance, it is most likely to be the hero or heroine who forces the other person to reevaluate their beliefs about what is important both in terms of the external story goal and in terms of their inner needs for fulfillment, no matter what the plot of your story is about.