Note: items are listed in order of gameplay.

Automatically obtained from the beginning of the game. See: Part 1: Reunion and Separation

In Underground Temple. Just before you come out of the underground, you will notice a large puzzle box. See: Part 1: Reunion and Separation

Solution: left, right, left

Inside a study room on the second of the Keep. The room is sealed with level-2 red crystal. See: Part 3: Trap, Trap, Trap!

At the corner of the door that is locked by the Crest Pieces is a ladder, keep climbing down twice. Inside the room is a treasure box containing the L. Crest Piece. See: Part 4: Rescuing the boy

This key item is optional. It doesn't effect your overall gameplay except giving you the ability to communicate with ghosts allowing you to learn more about the story as game progress. It is located inside the meat storage room, Keep's underground. By cutting the robe that ties the balance scale on the left side, the Rosary of Communication will be revealed on the right side of the balance scale. See: Part 5: Keep Underground

In a room next to the meat storage room in the Keep's underground, it is guarded by swinging blades. The bell is required continue the story. See: Part 5: Keep Underground

After you obtained Purifier Bell and use it to open a new path, continue to the next a few rooms until you see a door sealed with level-2 green crystal. The Blue Key is inside the big box. See: Part 5: Keep Underground

Automatically obtain from a dying soldier as Kaede. See: Part 6: Found the Princess

Wooden Gear is required to open a secret room to get another key item, Silver Plate. It is located at the west area after you walk pass bamboo garden. You will notice a flickering object on a dead soldier. See: Part 6: Found the Princess

After you picked up the Wooden Gear, go all the way back to the archery range, climb the ladder there and enter the room. Keep going until you reach a dead-end with a door. Inside is the room is a treasure box in the middle and cupboard on the wall. Put the Wooden Gear inside the cupboard and pull the string to open the secret room. Inside is the Silver Plate. See: Part 6: Found the Princess

The Gold Plate is in the furnace room, it is next the room where you get your Silver Plate. Though, you will have to open another door by pulling a string then pass 3 fire breathings from the furnace. The door will gradually close so you have to be fast enough. See: Part 6: Found the Princess

Evil Plate is key to open the door to the Demon World. After defeated the boss Stylado (Samanosuke's clone), proceeding the next area where you will find one chest and a box. The plate in the box. See: Part 7: Dark Realm

See the Evil Plate above, the ladder is in the other chest. I have no idea why the developers decided to put the Wood Ladder in a chest because its usage is very obvious. After you got the ladder, you use it immediately on the next screen to go up the well. See: Part 7: Dark Realm

Bishamon Ocarina is the key to open a secret room which contain Bishamon Sword, the ultimate weapon, in the Demon World. It is located in the 20th floor of the Dark Realm. 

After you got the Evil Plate, walk back to the Keep's hall and enter the sentry room, talk to the cocoon man he will invite you to the Dark Realm. Alternatively, you can find another cocoon man in a well in the west area. See: Part 7: Dark Realm

Vision Staff is an option key item, it helps you see hidden Fluorites. 

While you are on a mission to the west area, you will come across a room sealed with level-3 blue crystal. Inside the room, you will find the staff. See: Part 8: From West to East

Statue Head is another key to unlock your second long range weapon, Matchlock. It is located in a yard where you fight the red giant ogre (Barabazuu). See: Part 8: From West to East

It is required to unlock a door access to the east area. It is located in the same yard as the Statue Head (see above section). 

Well, I first thought it was the enhanced version of my normal bow. But it is actually a key item so you cannot use it as a weapon though, haha. It is later used automatically to destroy the seal that blocks you from enter the demon world. 

The Great Bow is located in a house on the other side of the river, so you have to use a boat from the storage house near the waterfall. See: Part 8: From West to East

Decorated Arrow is used as a decoy to replace Great Arrow when you pick it up. Because whenever you pick up the Great Arrow, it triggers a trap inside the room and lock you inside. That is why you use the Decorated Arrow to trick the trap.

When you are in control of Kaede for the second time, your first objective is to obtain this arrow. From the storage house near the waterfall, continue your way to the very end of the area until you see a ladder. The arrow is in a puzzle chest which is the hardest puzzle in the game. See: Part 9: A Helpful Partner

Solution: left, left, mid, right, right, mid, mid

The Great Arrow is in a room next to where you picked up Decorated Arrow. See: Part 9: A Helpful Partner

It is in the same room where you picked up Great Arrow but in a black treasure box. See: Part 9: A Helpful Partner