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.
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.
In a fanfiction where Tatsumi meets Esdeath early, one possibility is that Esdeath would immediately be intrigued by Tatsumi. She might see his potential and try to recruit him into her forces right away. Tatsumi, on the other hand, would likely be very cautious as he knows Esdeath's cruel nature. He might try to play along at first to avoid getting killed, but secretly be planning his escape.
One potential plot twist could be that Esdeath, instead of being hostile towards Tatsumi, decides to protect him. This would be a huge shift as she is known for her brutality. It could be because she sees something unique in him that makes her want to keep him safe, which would completely change the way Tatsumi views her and the power dynamics between them.
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.
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.