Jenna is not spoken to by name in the series, and appears only as one of Hillary's friends on the first day of school. She does have a brief speaking part in one scene.
Jenna's role is to go along with Hillary, including going along with rejecting Lori as those two are quarreling. When Hillary leaves the lunch table to sit with Declan, Jenna and the other girls get up too, leaving Lori alone and hurt.
When Hillary spills orange drink on Lori's blouse, Jenna is following behind, and avoids eye contact and laughing out loud.