Iris

Iris is the Goddess of the Rainbow. She carries messages for Gods, Goddess, and if you ask nicely and she's not too busy, to demigods. To Iris Message (IM) someone, you must find or make a rainbow (easier in modern times with the advent of the water hose) and throw a drachma through it, then say: "O, Iris, Goddess of the Rainbow, accept our offering." and name who you wish to speak to and where they may be found. She is the mother of Butch.

History
Iris is the daughter of Thaumas and the ocean nymph Electra, an Oceanid. Her sisters are the Harpies, Aello, and Ocypete. She also has a twin sister, Arkhe, who served as messenger for the Titans in the first war. As the sun unites Earth and heaven, Iris links the gods to humanity. She travels with the speed of wind from one end of the world to the other, and into the depths of the sea and underworld. Iris is married to Zephyrus, who is the god of the west wind. Their son is Pothos.

Percy Jackson and the Olympians
Iris is mentioned to be on good terms with Hermes, and always helps the demigods in moments of peril. Chiron also says he's a friend and she's known to do favors for him. A cabin is built for her demigod offsprings after The Last Olympian at Camp Half-Blood.

Throughout the series, Iris's services are called upon (by throwing a drachma into a mist) so that the characters may communicate with each other.

The Heroes of Olympus
Iris is the mother of Butch in the series. Even though Zeus forbade the gods to interact with the surface, Iris still sends Iris Messages for the demigods.(it is unknown why)