In The Tempest by William Shakespeare there is no romantic relationship between Caliban and Prospero. The love relationship in the play is between Fernando and Miranda. Although the play has certain elements of a revenge tragedy in Prospero's struggle to regain his dukedom, the happy ending means the actual plot arc is that of a comedy rather than tragedy. There really is no tragic plot line in the relationship between Prospero and Caliban.
For colonialism, often the play is considered to present colonialism in the relationship between Prospero and Caliban. Just as Europeans colonized less developed nations, so Prospero takes over an island that the "savage" Caliban should by rights have inherited from his mother. Caliban becomes a slave of Prospero, condemned to do menial labor. He is "civilized" by being taught the invader's language and expected to conform to the invader's social and moral code.