The messenger
Iris appears in Book II to summon the Trojan assembly, in Book III to bring Helen to the wall, and in Book VIII to carry Zeus’s warning to Hector and to turn back Hera and Athena. She moves faster than the wind and speaks with the authority of the god who sent her. Her messages are usually short and unambiguous.
She is not a character in the sense that Athena or Hera is. She has no independent plan and no comic subplot. Her function is to make the divine will present on the battlefield, and the poem uses her to keep the action moving.
The rainbow
Iris is the rainbow, a bridge between heaven and earth. In the Iliad she is sometimes sent by Zeus and sometimes by Hera, and the poem does not always distinguish between the two. Her appearances are visual as much as verbal: she arrives like a storm or a bird and delivers her message.
Her neutrality makes her a useful instrument for the poem’s plot. She can speak to either side, and she does not take part in the quarrels of the gods. She is the messenger as pure function, and her beauty is the beauty of a natural sign.