Secret Collectables

All secret items and the steps to attain them are listed below.

One of the earliest secrets in the game, ●EXCALIBUR● can illude many players on their first playthrough. To reach it, search for the rightside-up Mimiga Statue on the Underside, and examine it. A passage will appear, leading you to a hidden area on the right. The hidden area is known as the Sanctuary.
There are two puzzles in the Sanctuary. The first requires examining the Mimiga statue graves in order from left to right. The second is a much more complex 'Spirit Maze'. In this maze, a flame will follow your position around the room, and try to trap you. It's possible to lure the flame by moving to a position next to it.
In the final room of the Sanctuary, you will have to face off against an old boss. It is, ironically, easiest to fight this boss by using the ●FIREBALL●, since it will trick it into a pattern of jumping more often, where it will be easier to avoid its projectiles by moving left and right.
If you fail to reach the Sanctuary on your first trip to the Underside, it's possible to get there again. Speak with Jack in Grasstown after returning from the Labyrinth, then proceed through Grasstown until you reach the pit where you originally fell down. A familiar ghost will coax you into following it...
Crescent Medal

This is a test of skill. On your first trip to the Underside, proceed to the Sanctuary through the steps above, but DO NOT enter the door. Instead, touch the eye block to the left of the door, and chase it back towards the left.
Follow the block to the overhang area, then climb up and around to meet it on the other side, rather than riding it. When you reach where the Underside Shack is, ride the eye block until you reach the farthest left it will go. Enter the hole behind a fence near the start of the Underside to reach a hidden room with a chest.
Cat's Eye Medal

Defeat Monster X. This can be challenging depending on which weapons you have available to you, however it's possible to return to the Labyrinth at a later time for a rematch. Just make sure not to talk to him after winning, or he might steal the medal back.

It's easy to forget, but after you finish the Labyrinth, head to Sand Zone and give the ●ENERGY CELL● to Booster. He'll reward you with the ●AFTERBURNER●, an item that triples the rate at which the ●SNAKE● regenerates ammo.
The Spur

Any time after returning from the Labyrinth, visit the Gunsmith's house. He will be home from his wandering around the island, and ready and able to create the ●SPUR● for you, but you'll need the ●POLAR STAR● to do so. There isn't any information given beyond that, but it's implied that the ●POLAR STAR● is still obtainable.
After getting the ●BOOSTER v2.0●, head to the Mimiga Village Cemetery and check the door in the middle. Speak with the purple Pignon inside to start a fight for the ●POLAR STAR●! Knock out the green ones, then focus your damage on the purple one. Keep an eye on the green ones, as they'll get back up as the fight goes on.
Return to the Gunsmith's house with the ●POLAR STAR●, and you'll find Suto. The fight with him is broken up into two parts: first, he will jump around and throw missiles at you at varying heights. Next, he'll act the same as he did in Grasstown. Defeat both his patterns, and speak with the Gunsmith to obtain the ●SPUR●.
Whimsical Star

After both Genesis and Oblivion are defeated, you'll be forced to travel through the Revolutionarium again. Proceed as normal, and check the newly filled chest in the middle of the bottom room to find the ●WHIMSICAL STAR●! You might get in a fight with the Gaudskateers again, though.
Arms Barrier

After obtaining the ●MACHINE GUN●, collect enough EXP to level it up to 3, and jump down the pit on the far right of Grasstown. Break your fall by shooting the Machine Gun downward, and fly over to the left of the pit to find the ●ARMS BARRIER●.
It's possible to get this as early as the first visit to Grasstown. After you turn the Auxilery Power on, return all the way to the console on the far left of Grasstown and turn it off. Hug the left wall while falling down the pit, and the three fans that pushed you away will no longer be active.
Nikumaru Counter

Make your way roughly half-way up the Outer Wall, and keep an eye out for a rectangular block with a picture of a clock on its face. Follow the 'stem' holding this block to the wall, and examine the gap behind the stray railing to enter the Clock Room.
The ●NIKUMARU COUNTER● can be found in the middle. It will enable a timer under special circumstances near the very end of the game. Glory to all zealous challengers!