In The Tempest, Caliban is the local half-monster who was unfortunate enough to become a slave. Prospero was trying to civilize him by giving language lessons. However, the only thing Caliban actually wants is freedom and his rightful land. He even plots against his master, but the murder plan never works out.
The primary setting of the play is the remote island where all the magical spirits live. However, the only human inhabitant is Caliban, who was left to rule there after the death of his witch mother, Sycorax. Most likely, he was pretty happy there by himself until Prospero and Miranda arrived. The magician used his power to enslave Caliban and turned him into a servant. Prospero has never seen anything more in the islander than a savage who needs to be educated and civilized. Therefore, there were having some language lessons together, which Prospero considered being a nice gesture. Soon enough, Caliban realized that he far from what he expected. He was tired of being used and mistreated, so the desire to set free has begun to grow.
It all led to Caliban being bad-mannered and unfriendly, just as we see him in the play. Shakespeare often mentions him cursing Prospero and dreaming of killing him out of revenge. One time, the islander gets some support from Stephano and Trinculo, which sets them all on plotting against the magician. However, this attempt fails just like all the other ones Caliban had before. The only positive feature that this character has to offer is perhaps his love for the island. He describes it almost poetically and is ready to share all the knowledge he has about it.