Iris is the Greek goddess of the rainbow and the messenger of the gods, who carries oaths to the river Styx and serves as the bridge between Olympus and the world. Where Hermes is the cunning traveller, Iris is the arc itself: the visible road between two places.
Modern Practice
Iris is called on for safe messages, for reconciliation, and for any work that has to cross a boundary intact. Her practice is to say the thing accurately and deliver it, which is why she also swears oaths.