Oh, this is a can of worms, but I will bite.
The faithful slave is one of Jesus's parables. He uses parables to illustrate how men can please God.
The parable of the slave is so simple, to anyone who has ears to hear it. Like most of the parables, the meaning of this seems to be "hidden" from the wise and righteous ones who have declared themselves to have this special power of interpretation.
Anyone who serves the Master (Jesus), should consider himself to be a slave. Do what is pleasing to the master, and when he returns, he will reward you with more responsibility.
If I choose to serve Jesus, I am a slave. The idea of serving a slave, is absurd to me.
Where does Jesus tell us to serve the slave, even a faithful one?
(The JW's have mastered the art of confusion, and this is a prime example of their twisted teachings. Try explaining their belief to anyone unfamiliar with JW doctrine, and you end up sounding like a mental patient).