Rainy Brother and Rainy Sister appear on rainy days within the Midnight Channel, starting with the Steamy Bathhouse dungeon. They appear as a floating ball emitting red gases with an Arcana mask floating in front. They typically may only be damaged by their weakness and may escape if too many turns pass without defeating them. Selling only one of the materials they drop will unlock the associated item at Daidara.
Rainy Brother 4 may be found in Halls 1 and 2 of the Steamy Bathhouse. It may drop the 4th Boy's Talc when defeated. Selling this will unlock the Arazuyu Fan for Yukiko Amagi, which is the best weapon available to her during this dungeon if the Suzaku Feather was not acquired.
Rainy Sister 4 may be found in Halls 7 and 8 of the Steamy Bathhouse. It may drop the 4th Girl's Talc when defeated. Selling this will unlock the Reign Skinner daggers for Yosuke Hanamura in the shop.
Rainy Sister 3 is encountered on floors 1 through 4 of Marukyu Striptease. It may drop 3rd Girl's Talc when defeated. Selling a 3rd Girl's Talc will unlock the Akisame Fan for Yukiko Amagi in the shop, which has the highest attack rating for fans until the Secret Laboratory equipment becomes available, but only has a Hit rate of 50.
Rainy Brother 3 can be found on the 5th and 6th floors of Marukyu Striptease. It may drop the 3rd Boy's Talc when defeated. Selling this will unlock the Downpour Sword for the protagonist, which will boast the highest damage for several dungeons along with a heightened Critical Rate, but only has a Hit rating of 50.
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.
Rainy Brother 2 appears in the 7th and 8th Chapters of the Void Quest within the Midnight Channel on rainy days. After being defeated, it may drop 2nd Boy's Talc. Selling the 2nd Boy's Talc will unlock the Typhoon Claw weapon for Teddie, which has the highest Attack rating of weapons available for him at that point but a slightly reduced Hit rate.
Rainy Sister 1 appears on B1F and B2F of the Secret Laboratory within the Midnight Channel on rainy days. After being defeated, it may drop 1st Girl's Talc. Selling the 1st Girl's Talc will unlock the Niagra Climber male armor in the shop, which has moderate defense for Secret Laboratory armor and low evade, but comes with an Auto-Rakukaja effect.
Rainy Brother 1 appears on B1F and B2F of the Secret Laboratory within the Midnight Channel on rainy days. After being defeated, it may drop 1st Boy's Talc. Selling the 1st Boys Talc will unlock the Bloody Kunai weapon for Yosuke Hanamura in the shop, which has the second highest attack rating of weapons available from the Secret Laboratory and boosts the damage of his Wind spells by 25%.