In the play, Prospero is the exiled Duke of Milan who uses his magical powers to control the island and its inhabitants. Caliban is the native, deformed son of Sycorax, the previous ruler of the island. Prospero has enslaved Caliban and uses him as a servant, forcing him to do his bidding and obey his commands.
This relationship reflects Ngugi's ideas about the way in which colonizing powers have used mental control to subjugate native peoples. Prospero uses his superior knowledge and power to manipulate and control Caliban, just as colonizing powers have often used their superior technology and resources to dominate native populations.
Moreover, Caliban's deformed nature is symbolic of the way in which colonizers often portrayed native people as inferior and incapable of self-governance. Caliban's enslavement further highlights the brutal treatment and oppression that native people have faced under colonialism.
Through this relationship between Prospero and Caliban, Shakespeare presents a critique of the power dynamics and control systems inherent in colonialism, reflecting Ngugi wa Thiong'o's concerns and perspectives regarding mental control and the treatment of native peoples.