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.
One common theme is power. Aatrox is a very powerful character, so many fanfics explore his power, how he got it, and how he uses it.
The main characters are Brianna and Fiora, as the title implies.
There's also a fanfic called 'Aatrox: The Dark Warrior'. In this one, the author focuses on Aatrox's battles and how he overcomes his enemies. It's full of action and really exciting to read.
Aatrox was once a noble and powerful Aspect. But he was betrayed and corrupted. He was trapped in his own weapon, and his mind was filled with rage and a thirst for vengeance. Now he exists as a dark and fearsome champion in the League of Legends, seeking to bring destruction to those he deems responsible for his fall.
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.
In the Aatrox short story, he was created in a time of great need. He was meant to be a protector. But pride and the lust for more power crept into his heart. He started to see himself as above others. His actions led to his banishment or fall from grace. Now, he is a tragic figure. He is full of rage and pain, and his story is one of how even the mightiest can fall when consumed by negative emotions. His journey in the short story is filled with battles, both internal and external, as he tries to find his place in a world that no longer accepts him as a hero.