Protection of Cerpinus

Iku Nagae always speaks politely, and her personality gives off a hint of indifference. She also tends to find things bothersome. She believes that youkai that attack humans have no class and cuts them out of her life. She speaks in a way that exemplifies her haughtiness.

She serves the Hinanawi clan, who are celestials residing in Heaven, and acts as Tenshi's overseer. Tenshi has a boisterous and untethered personality, while Iku is more reserved and easily bothered. It's easy to think of this as a mismatch of responsibility, but there may actually be more to this pairing that is only known to the celestials.

Iku is Tenshi's sole overseer, and that job is unconnected to her role as the messenger of the Dragon Palace. Her original job was to spread the word of the Dragon God to humans. There are reports of Iku being sighted telling of coming calamities that have spread as folklore and legend.

There are many written myths in the world outside of Gensokyo. One such myth, Chilam Balam's Prophecy, talks about the celestial gods creating the stars and the dragons appearing soon after. Of those dragons, one kind was called Canhel, and Cerpinus (a Canhel dragon) cleansed the angels with water.

There is also one interpretation that instead of Cerpinus cleansing the angels with water, it instead spread the gods' teachings to humans. In this case, Iku's role is much the same as Cerpinus's. The two have such similar roles that it's difficult to consider it a mere coincidence...