The Hidden Heart is a Sword Kit from Iki Island. You win it by beating the Hidden Cove Tournament, the bokken contest at Fune's Refuge, and it changes your blades' look only.
Travel to Fune's Refuge on Iki and enter the Hidden Cove Tournament. Win every bout with the wooden practice sword and the kit is yours.
The same win opens the Crimson Dye Merchant's shop, so you get a new kit and a new vendor in one go. In Legends this kit needs a much harder unlock than the other Iki kits.
Helm of the Lower World
Helm of the Lower World is a helmet hidden on Iki Island. It sits in a secret storage room on the Mongol warship you dock during "Jin from Yarikawa", at Fune's Refuge.
To the Raiders of Iki Island
To the Raiders of Iki Island is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at inside the Mongol ship at Fune's Refuge on Iki Island, and it counts toward the Avid Reader trophy.
A Scrawled Message
A Scrawled Message is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a camp south of Lake Nagata on Iki Island, and it counts toward the Avid Reader trophy.
Attached to an Old Crate
Attached to an Old Crate is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a shed northwest of Lone Spirit Fall on Iki Island, and it counts toward the Avid Reader trophy.