Kaiden Musha possesses primarily Physical skills and a boosted Mahamaon, and is the next most dangerous target in the boss fight after Onnen Musha. Damaging it would prove risky as it can randomly reflect physical attacks with High Counter, while the Tenjin Musha possesses Makarakarn to reflect elemental spells. Fortunately, most party members will be outright immune to the element they cast, so pelting it with elemental spells would be a reliable means of consistently damaging it.
Rain End Musha is found in the 5th and 6th Chapters of the Void Quest as Shadows that only appear on rainy days. It repels all four elements (Fire, Ice, Electricity, and Wind), uses Power Charge to strengthen its attack (just like its Persona 3 counterpart), and possesses the passive skills Sharp Student and Null Dizzy. Also, while it is not strong against Physical attacks, they are slightly reduced by a hidden resistance modifier. It has a rare chance of dropping a Rain Steel when defeated. Selling Rain Steel at Daidara unlocks the Ama-Gakure male armor, which has the lowest Defense of armors available in the Void Quest but heightened Evade and a Agility boost of 10.
In Golden it appears on Chapters 3 to 5, 8 and 9. It may drop Howlite, Milky Quartz or Bloodstone for trade at Shiroku Pub.