Finding fanfics with 'lemon' content is not advisable as it goes against the rules of many platforms. However, if you're looking for regular, appropriate fanfics about Rentaro and Kisara from 'Black Bullet', you can check out websites like Archive of Our Own. They have a large collection of fanfics, but they strictly regulate inappropriate content.
I'm not sure where you can specifically find a 'Black Bullet' fanfic about Rentaro and Kisara with 'lemon' content (which often implies adult or inappropriate themes). Fanfics with inappropriate content are not allowed on many official platforms. You could try looking on general fanfic websites but be aware of their terms of use.
Rentaro and Kisara could be forced to participate in a dangerous tournament where they have to fight against other pairs. They train hard together, and during this time, their relationship deepens. They develop a unique fighting style that combines their skills and helps them win the tournament.
One possible fanfiction could be where Rentaro and Kisara go on a mission together outside the protected area. They face a new type of Gastrea that they've never encountered before. During the battle, Rentaro realizes his growing feelings for Kisara as he sees her fight bravely to protect him. In the end, they defeat the Gastrea and their bond grows stronger.
A possible plot could be Rentaro and Kisara going on a mission together to rescue some Civilians from the Gastrea. During the mission, they face various challenges and have to rely on each other's skills. Their relationship deepens as they overcome these obstacles.
Their relationship can be developed by having them share more personal stories. For example, Rentaro could open up about his past traumas regarding the Gastrea war, and Kisara could do the same about her family or her life before they met. This would create a deeper emotional bond.
One interesting plot idea could be a story where Rentaro and Kisara have to go on a secret mission outside the known safe zones. They face new types of Gastrea that they've never encountered before and have to rely on each other's unique skills to survive. Maybe Kisara discovers a new power within herself during the mission, and Rentaro has to figure out how to incorporate that into their battle strategy.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfictions for various fandoms, including Black Bullet. Just search for 'Rentaro x Enju' and you'll likely find many interesting stories. Another option is FanFiction.net, though the quality can vary more there.
One great Rentaro and Enju fanfiction is 'A New Day'. It explores their relationship in a post - apocalyptic world. It shows how they grow together and face new challenges. It has really good character development for both of them.
Some dedicated anime or manga fan communities might also be a good source. These communities often have their own sections for fan - created works, including fanfics about characters like Kenichi and Kisara. You can join these communities and ask around for 'kenichi x kisara fanfic' recommendations.
A great Rentaro x Enju fanfiction often has well - developed characters. It should stay true to their personalities from the original anime. For example, Rentaro's determination and Enju's innocence should be intact. Also, an interesting plot that explores their relationship further, like dealing with the challenges they face together in the post - apocalyptic world of Black Bullet.
A trope that I've seen a fair bit is the 'alternate universe' one. In these fanfictions, Rentaro and Enju are placed in different settings, like a high - school AU or a fantasy AU. It's really interesting to see how their characters adapt to these new environments while still maintaining the core of their relationship. It gives the writers a chance to explore different aspects of their personalities and how they would interact in different situations.