This is the Elemental Coin, key to unlocking the Elemental Axe and the object used to transform Luek into Nitro Mech, seen in the background.  He found this (not in a story) in an excavation site, and it prompted him to look for the Elemental Axe.  The Coin takes it's place on Luek's Chest when he transforms.

This is the Elemental Axe.  The Elemental Axe, when used with the Battle Mech (no pic available)  allows the Battle Mech to transform into the various elements in the universe (eg Earth, water, psychic etc) and the Axe's powers will change to reflect this. It was found at the North Pole of Mech, a cold place indeed.

The Great Battle Mech is a robotic suit that was built by Luek for the purpose of being Lueks second Nitro form (each Nitro has two)  Used in conjunction with the Elemental Axe, it will change it's form to suit the Element type.  (An offhand note, the Nitros have no dress code.  Just show up)

This is what Nicole uses to transform into Sailor Mech.  It was given to her on the first anniversery of her and Lueks marrige, by Luek.  Doesn't appear in Rogue Bull.