Realistically, their relationship could develop through mutual friends. If they are both invited to a party by a common friend, they might start chatting and find out they have some similar hobbies. Then, they could start doing those hobbies together, like painting. As they spend more time together in this way, their relationship would develop at a natural pace.
Their relationship could start with a shared goal, like protecting a particular person or place. Through working together towards this goal, they get to know each other better.
Their relationship could be developed through shared secrets. For example, Gumball discovers Carrie's secret fear of a certain type of supernatural creature that exists only in Elmore's underground world. He helps her face this fear, and in the process, they grow closer. They start to trust each other more and become better friends, which could potentially lead to a romantic relationship later on.
The characters' relationship could be developed through shared experiences with the lemon. For example, if they find a lemon that has a special power only they can understand, they'll have to rely on each other more, which would strengthen their bond.
Their relationship can be developed through shared experiences. For instance, if they get caught in a rainstorm together and have to find shelter. During this time, they start talking and realize they have more in common than they thought. It could also be developed through competition. They enter a local contest, and at first, they are rivals but then they see how the other person is also trying hard and start to respect each other.
First, start with their friendship. They are both members of Fairy Tail, so they would interact often. They could share their love for the guild and gradually start to understand each other on a deeper level.
First, they could start with mutual respect. Since both are likely survivors in a tough world, they would recognize each other's skills. For example, if the Sole Survivor is good at bartering and Boone is a skilled sniper, they can initially rely on each other for these specific tasks.
Their relationship could be developed realistically by having them work towards a common goal. For example, they might be trying to stop a new villain from taking over the world. As they plan and execute their strategies, they see the best and worst in each other. Gohan might see Android 18's determination even in the face of great danger, and Android 18 might see Gohan's selflessness. These experiences can bring them closer and form a relationship based on trust and admiration.
Their relationship could develop through shared experiences. For example, they both have had to deal with the fall of the Jedi Order in different ways. By sharing these stories and the emotions associated with them, they can form a bond. Also, if they are forced to rely on each other in dangerous situations, it can strengthen their connection.
Their relationship can start from a place of mutual suspicion. Given their different clan backgrounds and past interactions, this is a natural starting point. Then, small acts of kindness or cooperation can gradually chip away at the walls between them. For example, if they share a common enemy or a mutual goal like protecting a group of kits that are in danger.
To develop their relationship realistically, the author could focus on their individual growth and how they influence each other. For instance, Luna could help Harry to be more open - minded and less focused on the obvious. Harry, in turn, could give Luna a sense of security. They might start by having small gestures like Harry giving Luna a book she's been looking for, and Luna making Harry a special charm. As these interactions increase, so does their romantic relationship.