The Deadly Rival's Attire is Ryuzo's outfit, which Baku the Voiceless sells for 50 Ghost flowers in New Game+ only. It gives a massive health boost, major melee damage and stronger Ghost weapons, and it has no upgrades.
Start New Game+ and visit Baku the Voiceless on the island in the middle of Lake Izuhara, in Ariake. Buy the attire for 50 Ghost flowers.
If you only want the look, the 15-flower Kensei Armor dye is cheaper and keeps Kensei's upgradeable perks.
Aaruul
Aaruul is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at a camp south of Izuhara Clearing in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Adachi Dragon Mask
Adachi Dragon Mask is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Adak Tree
Adak Tree is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at Yoichi's Crossroads in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Adventurer's Fortune
Adventurer's Fortune is a dye for Traveler's Attire. It costs 10 flowers at the Izuhara merchants. Like every dye it only changes colours.