Richard Parker is also the symbolic manifestation of Pi's duality.  Yes, he has a civilized human side, but he also has a savage side that surfaces when survival mode kicks in.  Because this savage side is hard to accept or deal with, Pi creates his alter ego, Richard Parker.

I agree that Richard Parker is a manifestation of Pi's fears. But I believe he is also a representation of his strength. It is a strength he didn't know that he had until he had to face it, to turn his fear into power (i.e, taming Richard Parker). In a sense, Richard Parker becomes Pi's alter ego.

It is Richard Parker who kills Pi's dangerous "Friend"- something Pi himself could never do.

"This was the terrible cost of Richard Parker. He gave me a life, my own, but at the expense of taking one. He ripped the flesh off the man's frame and cracked his bones. The smell of blood filled my nose. Something in me died then that has never come back to life."

There are so many layers of meaning, and the name and character of Richard Parker has a long history in literature. Click here for some other perspectives that might illuminate how readers of "Life of Pi" might think of Richard Parker's role for Pi.

Richard Parker represents all that Pi fears, as well as how he overcomes those fears.  Pi must learn how to face his greatest fear of death, especially by a wild beast (he'd been told by his brother that one day he was going to be like a goat fed to a wild animal).  Using the skills he learned while helping at his father's zoo, Pi bravely faces his fear and masters it.  Using an analogy of a circus, Pi tames his own fears, and in doing so, survives against incredible odds.