Iris

Iris is the Goddess of the Rainbow. She carries messages for gods, goddess, and if you ask nicely and she's not to busy, demigods. To Iris-Message (IM) someone, you must find a rainbow (or some sort of mist) and throw a drachma through it, then say:

"Oh, Iris, Goddess of the Rainbow, accept our offering."

History
Iris is the daughter of Thaumas and the ocean nymph Electra, an Oceanid. Her sisters are the Harpies, Aello, and Ocypete.

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.

The series
Iris is mentioned to be on good terms with Hermes, and always helps the demigods in moments of peril.