There could be many stories about this 'mystery girl Olivia' in a Walking Dead fanfiction. Maybe she's a survivor who has some unique skills like being an expert at finding hidden supplies in the apocalyptic world of The Walking Dead.
One possible plot could be that Olivia is a long - lost relative of one of the main characters. She has been surviving on her own for a long time and when she reunites with the group, she brings with her a new perspective on survival. Maybe she has discovered a way to repel walkers that the group has never thought of before.
Michonne does die in the comics. The circumstances surrounding her death were quite dramatic and had a significant impact on the story's progression. It happened during a major conflict with other characters.