Guido Greco is the true identity of the body Mukuro Rokudo was possessing, to act undercover as Leonardo Lippi.
Guido Greco had killed 15 people before the age of 17 before he was convicted and jailed.
A year before Tsuna's arrival in the future, Guido escaped from prison. At an unknown time, Mukuro found and possessed Guido to be a spy to Millefiore Famiglia. A year later, one month before Tsuna's arrival, the Vongola caught him in a photo approaching future Chrome at an Italian airport in disguise while trying to locate Chrome's whereabouts after she disappeared. During this brief encounter, Mukuro instructed Chrome to go to Kokuyo Land.
He is tall and slender with short, dark hair and greyish-blue eyes. While in the Millefiore, he wears the standard Millefiore White Spell uniform. In his mugshots, he wore a purple jacket open over a light green shirt.
He replaced the real Leonardo Lippi, a short 60-year-old man who went missing, and takes his identity as an F-Rank communications officer. He works directly under Byakuran to gather information. During his time spying on Millefiore, he gave Byakuran false reports, one of such information was that the 11th Squad are busy in their struggle against Varia, so Byakuran will send Glo Xinia instead to Japan and give the latter information of Chrome's presence at Kokuyo Land in accordance to Mukuro's plan.
Later, just before the Merone Base Invasion, he is seen greeting Byakuran as he comes back from lunch. Suddenly, Guido-as-Leonardo timidly announced to Byakuran that he wanted to resign. Byakuran, however, unveils his identity as Mukuro, the Vongola Mist Guardian, who possessed Guido Greco and infiltrated the Millefiore. Byakuran reveals that he had already known for a long time since the meaning of the Datura flowers that used as decoration means "disguise" in flower language. With his cover found out, Mukuro then forms an illusion to transfer his full consciousness with his appearance shown as well to Byakuran, and begins the battle between him and Byakuran.
Later, after Mukuro is defeated, he attempts to leave; however, Byakuran had already enclosed the area in an invincible barrier and deliver the finishing blow. While later it was revealed that Mukuro survived the attack thanks to his ally's interference, Guido died from the last attack as Byakuran stated to Irie that he "might be in tomorrow's newspaper as a murder victim of something", implying that Byakuran had disposed of Guido's body after he killed him.