In a fanfic like this, perhaps there could be a situation where Tatsumi and Esdeath are forced to work together on a mission. During this time, they have to rely on each other and Tatsumi starts to admire Esdeath's strength and leadership skills. These qualities, along with her beauty, might draw him in and make him realize his love for her.
Another scenario could be that Esdeath uses her power and charm to pursue Tatsumi. She might be very direct in her affections, which at first makes Tatsumi uncomfortable. But over time, he finds himself falling for her unique personality. Esdeath could also protect Tatsumi from danger, which makes him see her in a new light and develop feelings for her.
Tatsumi might also face opposition from his friends and allies. They would likely be very suspicious of Esdeath and not understand why Tatsumi is falling for her. His friends could try to convince him that she's not good for him, which would put a strain on his relationships with them as well as his new - found feelings for Esdeath.
One possible scenario could be that they meet in a very unexpected place, like a small coffee shop during a rainstorm. Darien is immediately drawn to Serena's beauty and kindness. Another scenario might be that they are forced to work together on a project, and as they spend time together, Darien realizes his feelings for Serena are growing.
You can try looking on some fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you might be able to find Esdeath and Tatsumi - related ones there. However, make sure to follow the site's rules and regulations.
The popularity might stem from the complex relationship between Esdeath and Tatsumi in the original 'Akame ga Kill!' anime. Their dynamic, with Esdeath being a powerful and somewhat cruel character and Tatsumi being the more idealistic hero, creates an interesting contrast that fanfic writers like to explore further in a romantic or more adult - themed context (while still keeping it within the boundaries of fanfiction).
If Tatsumi is Esdeath's pet in fanfiction, there could be a storyline where Tatsumi is forced to participate in Esdeath's cruel games. He might be made to fight other 'pets' or perform dangerous tasks for her amusement. Or, perhaps Tatsumi tries to use his position as her pet to gain information and eventually turn the tables on Esdeath, leading to a plot full of intrigue and suspense.
Tatsumi would likely react with defiance. He's a brave character and wouldn't easily submit to Esdeath's kidnapping. He might try to fight back right away if he has the chance.
In certain fanfiction works, Tatsumi is indeed made into Esdeath's pet. This can be a way for writers to explore darker or more extreme versions of their relationship. It might involve Esdeath treating Tatsumi in a demeaning way at times, or it could also lead to a strange form of affection developing between them as the story progresses. It really varies from one fanfic to another.
One possible scenario could be that Ginny and Harry are in the Gryffindor common room. Ginny has had a long day of Quidditch practice and studying. She sits next to Harry on the couch, and as they are chatting softly about their day, her eyes start to droop. Harry notices but doesn't want to disturb her, and soon she falls asleep on his shoulder. Harry then just sits there, feeling a warm sense of protectiveness and contentment.
In this fanfiction, Tatsumi could be sent on a special mission that requires him to infiltrate Esdeath's inner circle. He disguises himself and gets close to her. Esdeath, who has a sixth sense for these things, suspects Tatsumi but plays along. As they interact more, there is an underlying tension between their opposing ideals, yet also an attraction that neither can fully understand or deny.
One possible plot could be that Tatsumi and Esdeath, after getting married, try to find a balance between their different ideals. Tatsumi, with his sense of justice, might influence Esdeath to be more empathetic towards the common people. They could face challenges from the political unrest in their world, and together they use their combined strength and influence to bring about positive change in their society. Also, their relationship could be a source of conflict among different factions who either oppose or try to use their union for their own gains.