Martina Crowe was a Messenger from the L.I.V.E. She became an Executive before Mr. Curtain was forced to close his "school". In the story, she passionately hates the members of the Mysterious Benedict Society.
In the 2nd book of the series, she seems to be the boss of the Ten Men. But in the end, Mr. Curtain leaves her while he escapes. Greatly angered by this, Martina tells Mr. Benedict all she knows about Mr. Curtain's plans. This could've been an advantage, but Mr. Curtain hadn't trusted her much, so he never told her much.
Martina Crowe did not appear in The Mysterious Benedict Society and the Prisoner's Dilemma, although she was mentioned a few times. She is taken into Mr. Benedicts home for questioning every so often, but otherwize is not mentioned in this book.
According to the illustrations of the Mysterious Benedict Society's companion book, Martina has a black ponytail and wears a green shirt and a pair of jeans and sneakers.
In the first book, she was described as a tall and athletic teenager with a shiny raven black hair.