Fairy Tales by A Brothers' Grimm

(Excerpt)

A certain king once fell ill and the doctor declared that only a sudden fright would restore him to health, but the king was not a man for anyone to play tricks on, except his fool .

One day, when the fool was with him in his boat, he cleverly pushed the king into the water. Help had already been arranged and the king was drawn ashore and put to bed. The fright, the bath and the rest in bed cured the diseased king.

The king wanted to frighten the fool for his act so he told him that he would be put to death. He directed the executioner privately not to use the axe but to let fall a single drop of water on the fool’s neck.
Amidst shouts and laughter, the fool was asked to rise and thank the king for his kindness.

But the fool never moved, he was dead- killed by the master’s joke.

Based on the relationships between the characters and the plot of this story, who is the only person who can play a trick on the king?