Spirit = Energy Bar
As Fenix's main weapon, Spirit would change colour during the course of the game depending on how much 'energy' it has. So instead of having a bar within the interface, you would see your energy levels in game.
(100%) (75%) (50%) (25%) (0%)
The colour would fade from dark grey/black to white, with light blue being the interchanging mid-colour - recharging every 5 or so seconds per stage. I feel this could add another dynamic to my game with minimal interface options.