It might be popular because it stays true to the essence of Aang's character. Fans who love the original show and Aang's pacifist nature would be drawn to it as it explores that side of him further.
Maybe it's a powerful jutsu or curse that doesn't eliminate the characters outright but instead forces them to find a way to adapt and survive, leading to interesting plot twists.
The 'aang doesn't kill fanfiction' could be unique in terms of the relationships it portrays. Since Aang doesn't kill, it might show how his friends and foes react to this. Maybe some friends admire his resolve, while foes might underestimate him at first. This can lead to complex relationship dynamics that are different from the typical action - based stories where killing is more common.
Build complex characters that have depth and growth. Their decisions and reactions drive the plot forward. Also, introduce unexpected twists to keep things interesting.
Their relationship could grow deeper as they face the challenges of pregnancy together. Krillin might become more in tune with 18's needs, and 18 could see a new side of Krillin's caring nature.
Bulma could also be a common character paired with Krillin. Bulma is very intelligent and could potentially help Krillin figure out how to time travel better or she might be the one who creates the time - travel device in the first place. And she could use her resources to help Krillin in his time - traveling adventures.
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