Is it possible to have more than one main character?
Question: I have a hypothetical idea for a novel. I say hypothetical because I don't know if I'll ever actually write it, but I'm enjoying planning the novel out.
I want to have three POV characters: the heroine, her love interest, and the secondary antagonist (he works for the main antagonist). The heroine and her love interest both have to decide whether or not to change over the course of the story, so technically they both are main characters.
Can having two main characters work? The hero and heroine are pursuing the same Story Goal, but in different ways, so can they be each other's impact characters? If the answer is yes, do I have to create separate storylines for each of them? And if the answer is no, is there a way for me to do this without sacrificing character development?
The short answer is yes, but there are some issues you may want to consider.
For instance, how much you develop each of your main (or point-of-view) characters is up to you. To develop each of them fully would mean giving each of them a personal arc in which they wrestle with a unique inner conflict. It might mean giving each of them a significant relationship with an impact character (who could be one of the other POV characters).
For example, it is common in romance to have the two romantic leads be each other's impact characters.
On the other hand, sometimes you may have a POV character who is only partially developed, perhaps to explain his/her actions, to create suspense (what will he choose to do?), or to give the reader an opportunity to find out something the main character
does not (thus creating dramatic irony).
As for whether you can develop a character without giving him/her a separate "storyline" I think the answer depends on how you understand these terms.
The best way to reveal a character is to show how that character wrestles with a dilemma and ultimately resolves it, and that means creating a dramatic arc. Having such an arc is close to the definition of a developed character.
Now, sometimes you can "develop" a character by presenting their backstory, which means showing how they wrestled with a dilemma in their past and how the resolution of that dilemma made them into the person they are when your story begins, but that's still giving them an arc.
And sometimes you don't present a character's backstory, because it's not relevant to your story (but it's good for you to know it, so you can write about the character more authentically). So that character may be "developed" in your mind, but the readers don't see it. Or the readers may get the sense that there is more to the character than meets the eye that may be the subject of a different story.
Perhaps you are wondering if a character's storyline has to be separate from the overall story or main plot, or if it can unfold at the same time? I would say yes to the latter. You just have to be careful about how you weave back and forth between the overall story and what's going on in the heads of your POV characters. You have to change perspectives at appropriate points so the reader can get caught up on what's happening with each character, without head-hopping within a scene.
Best of luck.