Matthew Vespucci is the son of Monica Vespucci and Robert. After the death of his father, Jean-Claude promises Monica that she and her son will "want for nothing". When his mother is away on business Matthew stays with Anita, Nathaniel and Micah, and has become very attached to them and some of the bodyguards. He calls them aunt and uncles despite Anita's initial reluctance. The week he spends at Anita's place in Dancing: An Anita Blake Novella is the longest he's ever stayed with them.
In Bullet, Matthew takes part in a dance recital of one of St. Louis dance schools. He is helped practice his moves by Nathaniel and Stephen, and during the recital both Anita and J. J. note that for a three year old he shows great talent.
In Kiss the Dead Matthew is mentioned briefly as a toddler they babysit sometimes. It disturbs Anita that Matthew insists on getting a kiss from her on the lips every time she sees him, "because all the big boys kiss 'Nita."
In Dancing Matthew accompanies Anita, Nathaniel and Micah to Zerbrowski's annual barbecue because Monica is on a work trip and it's a chance for Matthew to spend time with other kids outside of preschool and dance recital, and maybe even make new friends. He becomes a great success among the girls who dance, even those who are older than his four years, although it causes some strain between him and some of the boys.
One of the other boys attacks Matthew during the event, jealous of the attention he's getting from the girls, but the situation is resolved with some cold water and adult intervention. Later that evening the girls and Nathaniel set up an impromptu dance practice, with initially only Nathaniel and Matthew helping the girls show off their skills. The attacker and many of the other boys are drawn into the practice and get their own share of attention, and the hard feelings towards Matthew get pushed aside.
Matthew has deep auburn hair like his mother’s, but seems to share some of Robert's looks. When Matthew is four his curly hair has grown long enough to reach his shoulders, and Anita notes that the color is only a little browner than in Nathaniel's hair. Matthews eyes are a nice, solid brown.