She is played by actress Amanda Setton.
A member of Blair Waldorf's clique until she was dethroned as queen bee; she constantly shifts her allegiance between Blair and Jenny Humphrey. She makes no secret of her long time crush on Nate Archibald. She aids in Blair's dethroning and Jenny's ascent as Queen with Jenny earning her and Hazel's favor when Jenny salvages herself after Blair attempted to sabotage her birthday. She helps host Jenny's party with Asher and gives her the benefit of the doubt when she lied about having sex with Asher Hornsby, eventually ending her friendship with her when Jenny reveals the truth. She is also a national Merit Scholar. Chuck Bass persuades her to ruin Amanda Lasher's hair, similar to a stunt she pulled to another girl, which ends up in a fight between Dan Humphrey and Serena Van Der Woodsen. Isabel and Penelope contribute to the new rift between Blair and Serena when the position of Queen shifts from Blair to Serena. Penelope attempts to get Nate Archibald as her date to the Snowflake Ball, planning with the girls and a reluctant Jenny to humiliate Vanessa Abrams . Isabel, Penelope and Hazel are all present when they plan on declaring Emma Boardman as the new Queen, which went to Jenny. Penelope refuses to have a girl from Brooklyn carry out their legacy but Blair convinces them otherwise.
Penelope returns in season 4 as she attends Columbia University. She is currently a member of an exclusive club, the Hamilton House with Zoe, Jessica Leitenberg and Juliet Sharp, and continues being Blair's chief minion in her schemes.
She helps Beatrice Grimaldi get Blair wasted at her bachelorette party in Father and the Bride. She serves as one of Blair's bridesmaids in her wedding in G.G. After she's fired by Blair in The Return of the Ring, she helps Serena, who is in a war with Blair.
Penelope is a long time minion of Blair, but is not always shown to be very loyal to her. She's one of Blair's smarter minions, and doesn't seem to be as afraid of her as the other girls. In Blair's diary, she writes how Penelope is pathetic for always attending events alone.