Would her first appearance not be "Days Gone Bye", considering she was seen as a walker? She was first seen chronologically in the webisodes, but first appeared on screen in the TV show's first episode.
I love bicycle girl. She is my fave ZOMBIE
--The webisodes came out after the Episode itself, so it would be Days Gone By. The "First Appearence" is more for an out-of-universe reference than an in-universe one.