Dakuan is a spy for the Tokugawa clan who is opposing the Shogun of the dark's Toyatome clan. He is a shrewd and underhanded (although not totally evil) wicked little man. Underestimating him because of his size and age would be the worst mistake you could make. He, like Jubei, is very clever, but unlike Jubei, is a little sadistic and more willing to watch people die to achieve his own personal goals.
When Jubei gets poisoned, Dakuan could have told him how to recover... but he didn't. Instead he used Jubei's condition to force him into his employment. In fact, it's pretty certain that Dakuan threw the shuriken himself in order to insure Jubei's cooperation.
Dakuan also has a very interesting escape technique which he uses to elude the extremely dealy shadow dwelling demon Shijima. He camoflauges himself much like a chameleon, and clings to a tree and acts as one of its branches. Overall he isn't a very nice man on the outside, but at times he is very human inside. In the end Jubei finally gives him what he deserves. He believes that manipulation of people is the way of the ninja; Jubei's philosophy is no where near this.
Also an aside. This is pure opinion, but doesn't it seem strange that akuan knows about the Devils and has a strange power resembling something that could mark him as a devil? Just a thought, and it's never actually mentioned but is something to ponder.