Up+X to open, Down+X to pick up?
And what in the world is the key for?
A list of items and short descriptions would be ace.
However, as for getting the variety of a roguelike with the fun of a platformed
if there's a key in a level, there is also a locked chest.
I'll compile a quick list of items I know for you, while I'm at it:
Cape: press Z in mid-air to slow fall.
Eye of Udjat: let's you see jewels hidden in walls. will always be released form locked chests. I've never found two locked chests, so maybe the second gives soemthing else.
Climbing gloves: Useful! let's you cling to walls, and jump. basically, everywhere is an edge.
Jetpack: when I found this, I freaked. you can float for a short period of time. very useful. recharges on contact with ground. reminiscent of cave story's boosters.
there's also bomb paste, which makes your bombs stick to walls. it will always be dropped if you manage to kill a giant spider.