Mikasa and Eren don't kiss in the manga. The focus of their connection lies in different aspects of their shared experiences and the evolving plot rather than a romantic kiss.
Sure, Mikasa kisses Eren. This occurs during a crucial and emotional part of the story that has a big impact on their relationship and the plot as a whole.
Character development is key. It should show how their relationship has evolved over time to reach that kiss moment. For instance, if it's set in the original 'Attack on Titan' universe, it should account for all the hardships they've been through together.
Well, in fanfiction, the kiss between Eren and Mikasa can be a turning point. It might be written in a way that defies the original storyline a bit. For example, instead of the tragic events that unfold in the main story, the kiss could lead to a different path for them. It could symbolize hope for a more peaceful future together, away from all the chaos of the Titans.
In a more AU (Alternate Universe) setting, they could be in a world where the titan threat doesn't exist. They are just two normal people in a small town. Eren has been in love with Mikasa for a long time but was too shy to act. One day, during a local festival, he finally gathers the courage to kiss her under the twinkling lights.
For Eren, he could start off as a bit of a rebel in high school. Maybe he's always getting into trouble for questioning the school's rules. But as he spends more time with Mikasa, he starts to see the importance of following some rules and also becomes more empathetic. Mikasa, on the other hand, can be developed as the quiet, studious type who slowly comes out of her shell because of Eren's influence.