There's also 'The Unspoken Bond'. In this fanfiction, the author delves deep into the unspoken feelings between Kallen and Lelouch. It starts with small, subtle moments between them during the battles and gradually builds up to a more complex relationship. It's a great read for those who love to analyze the characters' emotions.
Another good one is 'Kallen and Lelouch: A New Beginning'. This fanfic takes place after the events of the series and imagines a world where they have to rebuild together. It focuses on how they support each other in this new phase of their lives, and how their relationship evolves from comrades to something more intimate.
One popular fanfiction might be 'A New World for Us' which explores their relationship in an alternate universe where the war ends differently. It delves deep into how they interact and support each other in rebuilding their world.
There's also 'Shirley's Choice' which focuses on Shirley's perspective in her relationship with Lelouch. It shows her internal struggle between her love for him and the complex situation he is in with his Geass powers and his goals. The story beautifully portrays her growth as a character while being in love with Lelouch.
One popular 'Code Geass Suzaku x Lelouch' fanfiction is 'A Different Path'. It explores an alternate universe where Suzaku and Lelouch's relationship develops in a more positive way early on. Another good one could be 'Shared Destiny' which delves deep into their complex bond and how they might face challenges together. There are also fanfictions that focus on their emotional connection during the war-torn setting of Code Geass.
In many fanfictions, their relationship starts from comrades in arms. They are both fighting for the same cause against Britannia. As they face more and more difficult battles together, they start to understand each other better and a deeper bond forms.
In many fanfictions, their relationship often starts from mutual respect. Kallen respects Lelouch's strategic mind, and Lelouch admires Kallen's strength. For example, in some fanfics, they start to rely on each other more during difficult battles, which deepens their bond.
There are fanfictions that explore their relationship from a more emotional side. For example, a story where CC reveals her past traumas to Lelouch, and he in turn shows her a side of himself that he has never shown to anyone else. This deepens their bond in a very different way compared to the main series.
One possible fanfiction could be a post - war scenario. After the chaos and destruction in the Code Geass world, Lelouch and Kallen, having fought side by side, realize their deep connection. They start to rebuild the world together and gradually their friendship turns into love. Eventually, they get married in a small, private ceremony attended by their closest friends and allies. The fanfiction could focus on how they overcome the differences in their backgrounds and the challenges of leading a new world while being in love.
Another scenario could be a goodbye kiss. If Lelouch has to embark on a very dangerous mission where he may not return. Kallen, realizing she may never see him again, impulsively kisses him. It would be a bittersweet moment filled with unspoken words and the weight of their relationship and the situation at large.
It could include their courtship. For example, Lelouch might start to show his softer side to Kallen, and she in turn would be more open about her feelings. They could go on secret dates in the rebuilt areas of the world, away from the prying eyes of the public. The fanfiction might also show how their friends react to their relationship. Some might be supportive, while others might be skeptical at first but then come around.
You can start by looking on fanfiction.net. It has a large collection of Code Geass fanfictions, including many focused on the Lelouch x Shirley relationship. Just use the search bar and type in relevant keywords like 'Code Geass Lelouch x Shirley' and you'll find a bunch of stories.