There is a link below to a related question, "What was a life for a serf?"
There is also a link below to a part of an article that deals with different kinds of peasants, including serfs.
Whats serf
Your question is not entirely correct in its premise; a serf is not a slave and does not have an owner. A serf is a subject of a land owner whom the serf would address as lord.
The homonym for "serf" is "surf."
In a feudal system, a serf would remain a serf no matter how hard they were to work.
Saint Serf died in 583.
Saint Serf was born in 500.
Riddle answer: a serf board.
Peasant/serf
The plural of the word "serf" is "serfs."
The noble executed the serf.
I doubt that they thought about the serf one way or the other. The serf was to do the work and didn't require much thought from his master.
A possible antonym of serf could be "free person" or "freeman," as a serf is a person who is bound to the land and essentially owned by a lord, whereas a free person has autonomy and is not bound to servitude.