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.
In Golden, it may appear in Chapter 6 through 9. It may also drop Howlite, Milky Quartz or Bloodstone for trade at Shiroku Pub.