In the manga, Eren and Mikasa don't have a romantic ending together. Their relationship is complex and goes beyond traditional romance. The story focuses more on larger themes and plot developments rather than a conventional pairing up of these two characters.
In a neighborhood, a girl, Emma, noticed a guy, Ryan, being mean to an old lady. Emma was into martial arts. She stepped in between Ryan and the old lady. Ryan tried to shove Emma aside, but she blocked his move and delivered a series of punches and kicks. Ryan ended up on the ground. Emma's actions made her a hero in the neighborhood and taught Ryan not to be so unkind.
Well, it could be about a scenario where Peter (presumably from some fictional universe) gets into a physical altercation with Flash. Maybe in this fanfiction, Peter has been pushed to his limits by Flash's actions or words, so he decides to beat him up. It could be a way for the author to explore the characters' relationships and emotions in a more extreme way.
Armin could be the storyteller. Sometimes characters take on this role to provide a unique viewpoint or to drive the narrative in a particular direction. But we need to know more about the story to confirm it for sure.
Armin is still alive in the manga. The story developments don't result in his demise. His presence remains significant and continues to shape the narrative.