Two of my favorite BtVS characters are Cassie Newton
and Dawn Summers.
For one thing, Cassie reminds me of me as I myself am a poet and a writer, and I sometimes post things online. Dawn Summers is a neat character, and after season 5 ended, she showed that, as a former Key, she has issues that all teenagers have. I think that both Cassie and Dawn are pretty cool characters to write fanfics about.