Number Two, or Pencilla, is Mr. Benedict's secretary and first adopted daughter. She is often portrayed as having a rather mouse-like character.
Number Two does not sleep, so food is her only source of energy. If she fails to consume enough food, she tends to get nervous and irritable. Her sleeplessness proves very useful on more than one occasion. For example, she manages to get a tremendous amount of work done on the eve of the Society's trip to the Institute, and in The Perilous Journey, her restlessness allows her to get the pin in her cuffs unlocked, and thus allowing her to escape. Her real name was not revealed until the end of The Prisoner's Dilemma, when Constance tricked her into thinking it, and plucked it from her mind.
Number Two has red hair, a yellowish complexion, and wears yellow clothing, making her look exactly like a pencil (her name and code name, Pencilla and Number Two, are likely a play on "#2 Pencil"). She is very tall and very slender, with small eyes that look rather observantly and nervously from one place to another, and are probably bloodshot as she "never sleeps." Despite her habit of always eating something, she is very skinny.
Number Two can be very defensive and closed off, especially about her name, which she considers embarrassing and goes only by her code name, which we know her as from the very beginning of the series. However, she cares very much about Mr. Benedict, Rhonda Kazembe, and Milligan, her adopted family. She is constantly worried about Mr. Benedict and always urges him to sit on the floor, due to his narcolepsy, and she doesn't mind lecturing him about it.
It is revealed at the end of the third book that her name is Pencilla, which "suits her." This is an obvious reference to her appearance.
Number Two is a technical genius and very fast typist. She can go for hours without sleep (which is why she is constantly eating) and she, like Rhonda and Mr. Benedict, has some magical quality that lets her put up with Constance every day.
Number Two was originally from Leadville, Colorado. She was orphaned as an infant. Her childhood is said to have been difficult—she was often ignored.
Number Two passed Mr. Benedict's tests wonderfully, but there were no other children to pass that year, and Mr. Benedict needed a team of children. He thought her talents so useful, though, that he hired her on as an assistant.