If you work her out decently, that feel of power will roll in itself. Let me give an example of a char concept I once made for a story, one that might bear some resemblance to what you're trying to make for Doku.
To make the history short; He's a man who never directly caused great evil, but he did so through inaction. He had the power and means to stop something and he chose not to, regretting this for the rest of his immortal life.
He exiled himself into the life of a hermit, travelling from village to village. Here he picked out the people who had it worst and played an innocent little trick on them. In the end, this trick rewarded them with money, goods or a magical blessing, whatever they needed most at that time. He'd disappear before they could personally thank him, taking his joy from seeing a smile on their face. It also sort of added to the spreading idea he was a spirit, or perhaps a small benevolent god.
Then, under the guise of something more powerful, he'd very rarely venture into courts and the homes of the rich and powerful. Preferably the ones who have people suffering under their leadership. He'd scheme up a trick for them that either lead them to see the error in their ways, or to their grim demise if they failed to see beyond their own pride. Their death was usually related to the type of pain inflicted on the populace, ranging from diseases to famine and worse.
There's a few differences with Doku's concept. He doesn't have a spirit he has to fight, instead he has to fight himself. He's in a constant internal struggle between making up for his mistakes and giving up to waste away somewhere. There's also a bit of evil leaking through in the form of murdering those he deems unworthy of ruling, comparable to the times when Doku loses it. He also uses tricks and power rather than his wealth alone to achieve his goals, but that's just a nuance.
Anyway, hope it gives you some inspiration.
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