All the things Fai is.
If Kurogane is like fire, then Fai is like water - dampening the flames. Blocking his furious advance, calming. Untameable, heavy, relentless - eroding barriers and slipping through the cracks.
If Kurogane is like fire, then Fai is like ice - warmed by the flames. Melting for him, soothing his fever. Delicate, cold, deadly - preserving the past and unable to move forward into the future.
Kurogane is fire, and Fai is ice.
Ice is both air and water, frozen in time. Unable to move, unable to break free - until fire unlocks time.