Key

A key goes hand in hand with a lite, because if you do not have a lite, you can not use a key.

Keys are discovered by fate, even an object you've never seen nor know how to use can become your key.

There is no specific kind of key, it can be anything. Most of the time however, it is a weapon.

That's pretty much all there is to it...

Examples:

Fiamn's guns, Nano's bow, Geers's video game controller, Malice's yaoi paddle