Katia Martelli is the forensics officer in Rome. She is seen as a romantic interest for Fabbri, but Rex is opposed to this. She is always looking for an opportunity to ensnare him. At one point, when Fabbri is injured and is at home researching a murder related to role playing games on his computer, he asks for a gaming police officer to be sent to his house to brief him about computer game culture. Martelli takes this opportunity to enter his house and seduce him, and creates an RPG profile for him so he can befriend the suspects online—the figure she chooses for him is a model of Fabbri wearing only a towel.

Morini constantly goads Martelli and Fabbri into coupling. After one case is solved, Fabbri takes her home to wine and dine her, much to Rex's chagrin. At the start of the next episode, they drunkenly awake in Fabbri's bed.

Since Lorenzo dies and is replaced by Davide Rivera, she appears with less frequency.