I don't have enough information to determine who Caroline is in this story. She could be an ordinary person with an extraordinary secret, or a fictional character created for a specific narrative.
Since we don't have much information about the 'captured caroline' story, it's hard to say exactly who Caroline is. She could be the protagonist, a victim, or a key figure in some sort of adventure or mystery.
Caroline as a human in fanfiction with Klaus can create a really interesting dynamic. It allows for exploration of how a human might interact with a powerful vampire like Klaus. For example, she could be a source of light in his dark world, constantly challenging his views.
Without more information, it's hard to say who Caroline and Nicolas are exactly. They could be ordinary people with an interesting story, or they could be characters in a fictional narrative.
Caroline Knapp is the author of this work. She is likely sharing her own experiences related to drinking in the context of what she describes as a 'love story'.
Another possible ally could be an older vampire who has a different view on the power struggles among the original vampires. This vampire might see something special in Caroline and decide to help her navigate the complex and often dangerous world of the original vampires.