Jillson is an Executive, rarely seen without her partner in crime, Jackson. She is a mean person and thinks that she is the best even though she really isn't. To prove it, she calls the Mysterious Benedict Society "little sqiurts." Jillson as an Executive obeys Mr. Curtain, and is a senior Executive.
Jillson is described as "a powerfully built woman, with a greasy brown ponytail and small, piggish eyes of an indeterminate color," in the first book. Later on in the series, she is said to be 6 feet tall, and have "arms like jackhammers." She is known to tie her ponytail with wire.
Jillson is often referenced as thinking herself superior to all others. It is later said that she holds special approval for Messengers, however.
She hates the members of the Mysterious Benedict Society with a passion, particularly Kate, who had embarrassed her the most at the institute. During her and Kate's first physical fight at the Institute, she had her ears painfully boxed by Kate in front of several other Executives, Jillson then proceded to threaten Kate and beat her until Milligan tranquilized her with his darts.