Colonnello is in charge of the training ground behind Mafia Land and was a former member of the COMSUBIN, an elite Italian Navy unit. Colonnello acts like a rival towards Reborn and vice-versa; they would often consider each other's attacks weak and then proceeds to attack the other. Though they are rivals, he shares many traits with Reborn, like being a merciless tutor, suddenly taking naps, and having a sharp tongue, he also finishes his sentences with a "hey" at the end.
Being a former military member, Colonnello is not averse to using heavy artillery; he was able to destroy an entire fleet of ships with one anti-tank rifle. This fits well with his name as it means 'colonel' in Italian. Also, an eagle on top of his head helps him fly to distant places in a short amount of time. It also seems to be in sync with Colonnello: when Colonnello naps, the eagle naps.
During the Vongola Tournament, he becomes Ryohei's home tutor and actually lives in his house. In the future, it is revealed that, as an adult, Colonnello followed the original seven Arcobaleno on the day that they were turned into babies in an attempt to take Lal's place. This resulted in him attaining the blue pacifier meant for Lal, and consequently turning Lal, who was his former instructor in the COMSUBIN, into an incomplete Arcobaleno.
SpoilerHowever, he is said to have died, with a Millefiore member, who was formerly known as "Ginger Bread", claiming that he died trying to protect Viper.