Short answers, pulled from the story.
Japanese armour first appeared in the 4th century, as evidenced by the discovery of cuirasses and basic helmets in graves. The earliest types, the tankō for foot soldiers and the keikō for cavalry, were constructed from iron plates connected by leather thongs.
The ō-yoroi was a luxuriously built, heavily constructed suit worn by high-ranking mounted samurai, while the dō-maru was a lighter style typically used by lower-ranking foot soldiers that fastened at the back. Both emerged during the Heian period (794-1185). By the end of the Kamakura period (1185-1333), even high-ranking samurai often preferred the lighter dō-maru.
The arrival of matchlock guns on the island of Tanegashima in 1543 prompted Japanese armourers to develop tosei-gusoku, a new style built from iron plates rather than individual scales. Bullet-resistant tameshi gusoku, meaning bullet-tested armour, was also developed so samurai could continue wearing armour on firearms-equipped battlefields.
Nanban dō-gusoku is a Japanese adaptation of Portuguese plate armour featuring an iron helmet and a solid cuirass. The first set was given to Sakakibara Yasumasa by Tokugawa Ieyasu just before the Battle of Sekigahara in 1600, and it was passed down within the Sakakibara family.
Ōyamazumi Shrine holds 40 percent of all armour designated by the Japanese government as National Treasures and Important Cultural Properties. Kasuga Grand Shrine also maintains a large collection of valuable armour.
The last documented use of samurai armour in combat was in 1877 during the Satsuma Rebellion. Armour had continued to be worn and used until the Meiji era in the 1860s before this final battlefield appearance.