To develop their characters, create situations where their values clash. Fiora values life and honor, while Aatrox may value power and domination. Have them in battles where they not only fight physically but also verbally, with each trying to prove their point. This will help to further define their characters and also show how they might change over the course of the fanfic.
For Fiora, you can focus on her noble upbringing and her sense of honor. Show how these traits influence her actions when she meets Aatrox. For example, she might be disgusted by Aatrox's use of dark magic but also respect his power.
Aatrox can be developed by delving into his past and his reasons for being the way he is. In the fanfic, when he interacts with Fiora, his character could evolve. Maybe he starts to question his own motives as Fiora challenges his views. You could also show his internal struggle between his dark nature and the small glimmer of something else that Fiora might bring out in him.
For Aatrox, you could focus on his past as a Darkin. His internal struggle between his dark nature and any glimmers of his former self could be explored. Show how Fiora's presence either exacerbates or soothes this conflict. For Fiora, her pride as a duelist is key. You can show how she reacts when faced with Aatrox's power that might be greater than what she's used to. Does she become more determined or is she momentarily daunted?
The plot might involve Aatrox being intrigued by Fiora's unwavering courage. He could see in her a potential to be a great warrior on his side, not as a minion but as an equal. So he tries to persuade her to join his cause, which leads to a series of intense dialogues and battles between them, as Fiora is hesitant but also curious about Aatrox's power.
One idea could be a story where Aatrox, the Darkin Blade, and Fiora, the Grand Duelist, are forced to team up. Maybe there's a greater threat to their world that only their combined skills can face. They start off distrusting each other, but as they face challenges together, they gradually come to respect one another's abilities.
The story could have Aatrox's ability to drain life force be a key plot point. Maybe he is weakened in the fanfic and needs to absorb life energy to regain his strength. Irelia's ability to control her blades could be used in a more creative way, like using them to form a shield against Aatrox's dark magic or to create a bridge across a dangerous chasm when they are on a quest together.
To develop Ahsoka's character, show her struggle with her identity outside of the Jedi Order when she's with Padme. Maybe she's trying to find a new purpose. Padme can be the one to encourage her to use her skills in different ways. For Padme, show her more vulnerable side. When she's with Ahsoka, she can let down her political facade a bit. They can have heart - to - heart conversations that allow the reader to see deeper into their characters.
For Smoker, you can show his growth in terms of his leadership skills. Maybe at the start of the fanfic, he is a bit too head - strong. But as he works with Hina, he learns to be more diplomatic. For Hina, her character can be developed by showing her softer side. Usually she is seen as strict, but in the fanfic, she could show kindness towards Smoker which is a new side of her.
For Bunnicula, you can develop his character by exploring his backstory. Maybe he was once a normal rabbit but was turned into what he is now. This could add depth to his character. As for Chester, show his relationship with the other pets in the house in more detail. Have him interact with them in different ways. In the story, let Bunnicula teach Chester about his world, which will make Chester more understanding and less paranoid. Also, give them both goals. Bunnicula might want to find a way to be more accepted in the household, and Chester could aim to be a better protector.
Esdeath's character can be developed by exploring her backstory in relation to the thief. Maybe she had a similar experience in her past where she was an outcast like the thief. This can make her more empathetic towards the thief. The thief's character can be developed by showing their growth from being a simple thief to someone who has a greater purpose, perhaps influenced by Esdeath's grand plans. As they interact, both characters can change and adapt, making their relationship more complex and interesting.
Luma's character can be developed by highlighting its curiosity. It's new to the universe in a sense, so it can constantly be asking questions, exploring new things. You can also show its loyalty to Rosalina, like how it always stays close to her and follows her instructions without hesitation. As the story progresses, Luma could grow in confidence, starting from a bit of a timid character to one that can face challenges on its own, with Rosalina's teachings in mind.
The theme of self - discovery is also quite common. Through their relationship, both Fiora and Riven may learn more about themselves. In some fanfics, Fiora discovers a new side of herself as she gets to know Riven better. And Riven may come to terms with her past and her identity through her bond with Fiora.