Apparently, Rep. Marie St. Fleur is Tom Reilly's pick for Lieutenant Governor. I don't actually know that much about her, so I thought I'd see what I could quickly dig up.
The answer is: not much. She is apparently the first Haitian-American ever to hold elective office in Massachusetts. She is also an experienced prosecutor, having started her career at the Middlesex District Attorney's office in 1987 (under then-Middlesex DA Scott Harshbarger) and moving to the Attorney General's office in 1991 (the same year that Harshbarger became AG). She has served in the House since 1999 and is now Vice-Chair of the House Committee on Ways and Means.
That's about all I can find (mostly from this brief bio). Anyone out there got more info?