Richard Parker represents Pi's will to survive because it is only through the tiger that anything necessary to brutal survival comes to pass. Wild animals learn to survive each day by doing that which is necessary. If what is necessary means killing and eating another species, then so be it. For Pi, a very religious and spiritual boy, killing other animals for his own survival begins traumatically. However, if Pi can associate his will to survive with that of a wild animal, who has the excuse to act brutally for his survival, then it is easier for the boy to accept it himself.
One way to associate and understand Richard Parker's connection to Pi's will to survive is to substitute "will to survive" with "Richard Parker," as in the following example:
"It was Richard Parker who calmed me down" (162).
The substitution would read, "It was my will to survive [that] calmed me down." With this technique, one can see how Pi would be able to accept himself as a human if he also applied his animalistic tendencies for survival to a tiger.
Another example could be used when Pi throws the rat at Richard Parker and the tiger eats it in one gulp, as follows:
"I suppose I was partly responsible for the rat's death, but I'd only thrown it; it was Richard Parker who killed it. A lifetime of peaceful vegetarianism stood between me and the willful beheading of a fish" (183).
The substitution would read, " . . . it was my will to survive [that] killed it." In this case, Pi must survive by killing animals, a practice that he never would have done because of his peaceful belief that all life matters. It's as if Pi creates Richard Parker in order to excuse or justify his will or need to survive at sea.
Finally, Pi promises the tiger that he will save him by making sure that they both make it to land safely. Pi gives Richard Parker a pep talk in the following passage:
"I love you, Richard Parker. If I didn't have you now, I don't know what I would do. I don't think I would make it. No, I wouldn't. I would die of hopelessness. Don't give up, Richard Parker, don't give up. I'll get you to land, I promise, I promise!" (236).
In this case, no substitution is necessary. If one simply realizes that the tiger is Pi's alter ego, then it is easy to see that Pi is making a promise to himself that he will survive. Not only is he making a promise to himself, but he is also using the illusion of the tiger to encourage himself and to take responsibility for his own (as well as the tiger's) survival.