To develop Krillin further, fanfiction could show his role as a mentor to younger fighters. He could pass on his knowledge and experiences. Android 18 could be developed by exploring her relationship with other androids. Maybe she tries to convince them to side with the good guys. This would show her leadership skills and her ability to change the minds of those around her.
For Krillin, he could be developed by showing his growth in confidence. Maybe he takes on bigger threats without hesitation. For Android 18, her softer side could be explored more, like her maternal instincts if they have a child.
In fan fiction, their characters can be developed by giving them new goals. Android 18 could aim to become a better mother to Marron, and Krillin could strive to be a more respected martial artist in the community. They could support each other in achieving these goals. Android 18 could use her strength to help Krillin train, and Krillin could offer his wisdom to Android 18 in parenting. This would add new layers to their characters and their relationship.
Krillin could be developed by showing his growth from being a bit of a coward sometimes to a more confident fighter because of his relationship with Android 18. Android 18 might be developed to show a softer side, especially towards Krillin. For example, she could start to care more about his well - being and show emotions that she previously hid.
To develop Android 17 further, you could explore his relationship with nature more. Since he has that love for animals, show him trying to protect a large area of wildlife from human destruction. This would not only show his growth in his beliefs but also his ability to take on big challenges.
For Alaric, his character could be developed further by exploring his past traumas more deeply. Maybe he has some unresolved issues from his time as a vampire hunter that resurface when he's around Caroline. For Caroline, her character could grow by having her face more complex moral dilemmas. For example, if she has to choose between helping Alaric or following the rules of the vampire community. They could also develop their relationship by having them share more personal stories and secrets with each other.
For Boruto, he could be developed by facing more complex moral dilemmas. For example, having to choose between the safety of the village and the life of a friend who has been misled. Mitsuki, on the other hand, could be developed by exploring his origin more deeply. Maybe he discovers more about his 'parent' Orochimaru and how that affects his identity as a ninja.
Bart's character could be developed by making him more responsible. In the fanfic, Greta could be the catalyst for this change. She could give him tasks to do that are important, like helping her with a community project. As he does these things, he realizes he can be more than just a troublemaker. Greta, on the other hand, could be developed by showing her flaws. Maybe she's overly perfectionist and Bart helps her see that it's okay to make mistakes.
For Ardyn, dig deeper into his emotions. Show his vulnerability when it comes to Aera. Perhaps write about his dreams and nightmares related to her. In terms of Aera, develop her character by showing her influence on the people around her. You can also give her a more active role in the events that unfold, rather than just being a passive love interest. Let her be the one who tries to change Ardyn's fate actively.
Character development could occur through their interactions with other characters. If they meet new androids or humans, how they react can show different sides of their personalities. For instance, if they encounter a human who is prejudiced against androids, RK800 might try to reason with the person, while RK900 could be more defensive. This would show RK800's more diplomatic side and RK900's protective nature.
One way to develop Toshiro's character in fanfiction is to give him a moral dilemma that challenges his beliefs. For instance, he might have to choose between following the strict rules of the Soul Society or saving Momo in a situation where the rules might not allow it. This would force him to reevaluate his loyalty and values. As for Momo, her character could be developed by delving into her past traumas. Maybe she has some unresolved issues from her childhood in the Soul Society that are affecting her current relationships. By facing these traumas, she can become a more complex and well - rounded character, which in turn would impact her relationship with Toshiro.
There are likely some fanfics about Krillin and Android 18 that focus on their relationship in a non - sexual and family - friendly way. You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own. Search for stories tagged with 'Krillin' and 'Android 18' to find ones that might interest you.