Basic: The Arcobaleno are adults stuck in toddler bodies. They still think like adults, and their skills are those of their adult selves.
Each Arcobaleno was given a colored pacifier, to signify their being Arcobaleno. Each pacifier glows whenever another comrade is in the immediate area. They are called the Arcobaleno (Italian for “rainbow”) because each pacifier is a different color of the rainbow.
EVERYTHING BELOW IS SPOILER FOR FUTURE ARC. If that does not worry you, keep reading.
In a mysterious chain of events, seven of the best people within the Mafia (from Vongola and from other branches) are converted from adults to babies. The purpose and the process of this conversion is still not known, but it has to do with protecting the ultimate power.
It has also been hinted that each pacifier holds a lot of power. The seven pacifiers are included in the Tri-ni-sette that Byakuran wants to complete.
Recent manga developments have given the names of all the Arcobaleno:
(Clockwise from helmet: Skull, Fon, Viper, Verde, Reborn, Colonnello, Luce )
|Luce||Orange||Sky||Squirrel (Cosmo)||Giglio Nero|
|Red||Storm||Japanese macaque (Li-chi)|
|Viper (Mammon)||Indigo||Mist||Frog/Ouroboros (Phantasma)||Vongola Varia|
* Luce is Uni’s grandmother.
* Fon is clearly NOT Hibari, but it is also clear that he has links to Hibari of some kind. Again, we know nothing more for now.
* Lal Mirch is ALSO Arcobaleno, and should rightfully hold the Blue Pacifier, but Colonnello took her place, thus the process ran into problems with her. Her pacifier is thus black, and considered corrupted. It is the reason why she was also susceptible to radiation effects like all the other Arcobaleno, and she has grown older over the ten years between the Varia and Millefiore arcs.
(Elementals information given by nonoji with thanks.)