The OC could be a mysterious figure who has some sort of psychic ability. Paul Rovia is initially skeptical but as they face various threats in the Walking Dead world, her ability proves useful and they grow closer. For example, she can sense when walkers are approaching from a distance. This unique aspect of her character can drive the plot forward in the fanfiction.
To create a strong OC, think about her motivation. If her main goal is to find her lost family, it gives her a strong drive. In relation to Paul Rovia, she could initially see him as an obstacle to her goal because his group has different priorities. But as they interact more, she realizes they can help each other. Her determination to reach her goal, along with her evolving relationship with Paul Rovia, can make her a very interesting and strong character.
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.