The Kralahome is the Prime Minister of Siam and a very powerful advisor, sorcerer and a diplomatic figure. Also, describing his personality, he is evil, treacherous, villainous and antagonistic (towards Anna).
He has designs to taking over the rule of Siam. Under his rule, things would have been backwards, not forwards. The people would be ruled by superstition and fear, just the kind of atmosphere that makes people fall prey to his dangerous sorcery. But he first get rid of King Mongkut by overthrowing him.
His plan is to prove to the British that King Mongkut is a barbarian and should be removed from the throne. Someone more in touch with what the people really need should be put in his place, someone like him.
Kralahome is an evil sorcerer whose main goal and plan is to become king of Siam by overthrowing King Mongkut by sabotaging his reputation a good leader. He believes if harm is done to Anna Leonowens, Louis, Tuptim, and Prince Chulalongkorn then King Mongkut would be considered to be a horrible person and that he would be eliminated from the throne. He could then proceed take over the kingdom and rule it for himself.
Master Little does not feel appreciative of Kralahome as his henchman, hence calling him "stingy" by not sharing the wealth when he becomes king. Kralahome has the power to create illusions with the help of a magical gong that can see all as sort of like a crystal ball and mind powers.
Towards the climax of the film, Kralahome is foiled by Anna's partner, Sir Edward along with the guards for trying to kill King Mongkut, and his plans were uncovered. After he was arrested, his punishment would be permanently working the elephant stables by cleaning their dung with master Little as his boss. He then gets beaten up by Master Little after getting angry about his last tooth falling out.