Penny Carter

Penny Carter was a journalist working as science correspondent for The Observer. She attended a press conference at Adipose Industries; very sceptical about the diet pill, and decided to investigate it a bit further. She stayed at Adipose Industries till after closing time, hidden in a toilet stall, but was discovered by Miss Foster - thus saving Donna, who was hiding in a stall nearby as part of her own investigations into the company.

Miss Foster took Penny to an office, and her security men tied her into a chair. She was later released by the Tenth Doctor and warned to get out, but stayed behind to collect files for her story and ended up getting tied up again. When Miss Foster set the Inducer, she told Penny to mark the date: "Happy birthday; so many birthdays". After the March of the Adipose, Penny escaped (still tied to her chair) and told the Doctor and Donna Noble that she would report them for "madness"; Donna pointed out that "some people just can't take it" (alien intervention), to which the Doctor agreed. (TV: Partners in Crime)

Behind the scenes
In his early notes regarding the character of Penny (reproduced in The Writer's Tale), Davies described her as being in her 30s and Donna Noble-esque, even a bit of a klutz.
 * According to The Writer's Tale, before Catherine Tate agreed to reprise her role as Donna Noble, Russell T Davies wrote a character called Penny Carter to be the Tenth Doctor's new companion beginning with Partners in Crime. This character was scrapped when Tate agreed to reappear as Donna, and "Penny Carter" was later used as the name of the journalist who was investigating Adipose Industries.