At first, their relationship might be full of tension. Demetri is a vampire and Leah is a werewolf. But as they interact more, they could find common ground in their loyalty to their respective groups. They might start to communicate secretly to avoid causing chaos between vampires and werewolves.
Their relationship could develop in a rather slow - burning way. Demetri is used to the vampire world and Leah to the werewolf world. They could start sharing their experiences, like Demetri telling Leah about the Volturi and their strict rules, and Leah sharing about the pack dynamics. This exchange of information could lead to a deeper understanding and eventually a closer relationship. For example, they might start to protect each other from threats within their own groups that don't approve of their friendship.
If Demetri and Bella had a relationship, it would surely create a rift between the Cullens and the Volturi. The Cullens would be very protective of Bella and not trust Demetri easily. The Volturi, on the other hand, might see this as a threat or an opportunity depending on how they perceive Demetri's actions.
They also use dialogue to build their relationship. For example, Jacob might share his dreams and fears with Leah, and she in turn reveals her own insecurities. This exchange of personal thoughts creates a deeper connection between them. Writers also add in little details, like how they look at each other or small gestures of kindness that gradually grow into something more significant.
One idea could be Demetri and Leah teaming up to protect the Forks area from a new threat that neither the vampires nor the werewolves have faced before. They might start off as reluctant allies but gradually develop respect for each other.
Their relationship could develop slowly. At first, they might be just comrades. But as they share their past traumas and experiences, they begin to understand each other on a deeper level. For example, Leah might open up about her struggles in her pack, and Kakashi could talk about his losses. This emotional sharing would bring them closer and turn their relationship into a more intimate friendship or even something more romantic over time.
Their relationship could develop through mutual respect for each other's power and knowledge. At first, they might be a bit wary of one another, but as they face challenges together, they start to trust each other more. They could share stories of their long lives and find common ground in their experiences.
Their relationship could start off rocky. Jacob has always had a bit of a temper, and Rosalie is not one to back down easily. But as they interact more, they might find common ground. Maybe it begins with a grudging respect for each other's loyalty to their own kind. As time passes, they could start to see the good in each other beyond their species differences. For example, Jacob might admire Rosalie's fierce protectiveness of her family, and Rosalie could come to appreciate Jacob's straightforwardness.
The popularity might also stem from the fact that their relationship, as shown in the source material, might be ambiguous. Fanfiction writers can then explore the possibility of a romantic connection between them through a kiss. It gives fans a chance to see their favorite characters in a new light, in a romantic situation that wasn't fully explored in the original work.
Often, their relationship becomes more complex. They might start off as just friends or comrades, but the pregnancy forces them to become closer, more like a family.
In an alternate universe, their relationship could start as a friendship based on mutual respect. Felix might admire Bella's courage in the face of danger, and Bella could be intrigued by Felix's strength and loyalty to the Volturi. This friendship could gradually turn into a deeper emotional connection as they face various challenges together.
If they were in a modern city setting, their relationship might develop in a more secretive way. Jacob would have to be careful not to shift in public, and Renesmee would have to hide her vampire - like abilities. They could meet in cafes or at the movies, trying to have a normal relationship while constantly being on guard. This would add an element of mystery and danger to their relationship.