Rainy Sister 2 appears in Chapters 3 and 4 of the Void Quest within the Midnight Channel on rainy days. It may drop the 2nd Girl's Talc when defeated. Selling the 2nd Girl's Talc will unlock the Amagyou Fan for Yukiko Amagi, which may be considered the best fan weapon in the Void Quest unless one prefers the Akisame Fan which has higher attack power but lower accuracy.
In Golden, it also appears in Chapters 8 and 9. It may also drop Howlite, Milky Quartz or Bloodstone for trade at Shiroku Pub.