She is actually considered as a neighbor in the game, because the player can easily customize her appearance in the main menu. Aside from housesitting for Doctor Newlow, she has no other purpose in-game. The player may socialize with her if they wish.
Her file may be messed up because her thumbnail shows her with a black cowboy hat and blonde hair. In Create-A-Sim, under the "Customize Neighbors" option, Penelope appears to be blonde if the player selects her.