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.
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.
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.
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.
I'm not sure specifically about 'lemon' fanfiction as it often involves inappropriate or adult - themed content. However, there are many regular Tatsumi and Esdeath fanfictions on various fanfiction platforms. You can try Archive of Our Own or Fanfiction.net and search for their relationship - based stories that are more in line with general storytelling and character exploration.
One common theme is the struggle with their different personalities. Tatsumi is often portrayed as more kind - hearted, while Esdeath is cruel. So when she becomes pregnant, there might be a theme of her learning to be more empathetic. Another theme could be the protection of the unborn child, with Tatsumi taking on a more protective role.
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).
It might not be so straightforward. While Esdeath has the potential to be the antagonist, there could be other forces at play. For example, there could be a third party that is manipulating both Tatsumi and Esdeath for their own gain. In this case, both Tatsumi and Esdeath could be seen as victims of this greater evil, and they might have to team up against this new threat. However, if we consider the traditional view, Esdeath's actions and beliefs are in direct conflict with Tatsumi's, so she remains the likely antagonist.