Storms and associated Water Magic would be the start. Rage Powers of any kind would fit the deity pretty well. He's kind of a force of Chaos, and combined with the water element already in the Storms aspect I could see Entropy Magic playing a roll
the Kami fought an eight-headed dragon, and found a sword in its tail which became part of the imperial treasures, last seen in the early 1990s. this could be a potent item of power that shows up later on.
According to the books that particular sword was one of the Swords of the Cross...
Could be interesting to give them a Serpent affinity, the ability to command/intimidate Serpents into doing their bidding. Snakes wouldnt like him, and would probably resent him if they are intelligent enough for such things, but they'd be subject to his will unless they are supernatural and clever enough to slip out of it.
It's a fairly small benefit. Easy thing to do would be to swap in some negative- they have an aquatic nature now, so in areas which are extremely dry magic is harder. Dry air could serve as a threshold.
I'd hesitate to use that specific one, as dry air is often a benefit to static electricity and lightning, so should be a beneficial thing for the Storm aspect, not a restriction. Being a more "stormy primal chaos" entity might weaken their provided magic in City's and other area's of overly "ordered." Deserts and other arid places of course. Or you could flip it and say that, rather than having difficulty with Water because it's grounding their magic (like normal) they have difficulty with Open Water (and maybe heights) because it calls to their Storm Magic and is more difficult to keep under control in area's of strong resonance, rather than weaker in opposite areas.
Check out the Suosano entry from the
Scion wiki. Different system of course, but its a concise writeup of the the mythological and personality traits of the deity.