I'm the author of an ongoing series called Versatile Layer. So far, I have 5 books released with the 6th nearing completion. With the way the story is moving along, it should wrap up in book 8. Versatile Layer can best be described as Romeo and Juliet meets Star Wars. Our main protagonists, Jake Takeda and Adeola M'falme, are from completely different worlds and cultures. Jake is a Terran (human) and Adeola is an Arez, a race of tall red skinned aliens who are physically superior to humans. Fate thrusts them together and regardless of their differences, they form a unique bond. But as fate would have it, their two races have been warring off and on with each other for generations. There's an intense hatred between their peoples.
They're separated by the end of the first book and the rest of the series follows their individual journeys. They grow and face adversity. All the while, longing to find each other again. Each book experiments with different genres also. The first book is a coming of age tale, the next is military action set on a distant moon, followed by a martial arts tale set on an alien world, and so on. The series is a love letter to all of the genre cinema that I enjoyed since childhood but also a deconstruction of it at times.
Know your characters. Especially little insignificant details about them that will never even be mentioned in your book. I can tell you what each of my main characters' favorite foods are. It doesn't matter to the narrative but the more you know about your characters, the more alive you can make them feel to the reader.
Also, every once in a while, let a character do a thing that doesn't affect the overall story. In life people express themselves in ways that don't matter at times. In ways that only matter to them. If you know your character, then when they're random, you'll know why they did that random thing and the audience will accept it because you as the writer put it in with confidence and purpose.