Sei does not utilize Willpower as a weapon in the way the rest of the cast does. Willpower can be used as if it were second-nature and can be used easily.

But this is not the case when Sei shifts into Nekomata form. Using built up Malice, Sei allows negativity to overcome himself and grant him devastating strength and agility.

In exchange, Nekomata is terribly dangerous because Willpower is not the source of its strength. Kalen is capable of preserving the environment, because his pyrokinesis is Will-powered; he can prevent it from damaging things he is not directly targeting. Whereas whatever is in Nekomata's way is at its mercy.