What is the "essence" of Pi in Life of Pi?
As a young man, prior to the shipwreck, Pi is interested in science, zoology, and religion. He is so open-minded that he finds value in multiple religions. Having these interests, Pi is essentially looking for truth, materially and spiritually. This is important for the entire novel. Pi's ability to combine religion and science allow him to make sense of the world in meaningful ways.
When the boat sinks, Pi must adapt to his new way of life: survival. Remembering how some animals could live together in the zoo, he learns to adapt this to his own life on the boat in similar ways. He had been a vegetarian before the wreck, but had to compromise that and eat meat. When Pi finally makes it back to society, again he has to adapt to living in the civilized world.
As Pi adapts to each dramatic change in his life, he must make sense of it. Pi uses religion and science, not in opposition, but in a synthesis which he uses each time he adapts to a new situation. This speaks to one of the themes of the novel which is that there is as much truth in a parable as there is in the actual circumstances which provide the basis for that parable. Or, as Pi says to his interrogators:
So tell me, since it makes no factual difference to you and you can't prove the question either way, which story do you prefer? Which is the better story, the story with animals or the story without animals?
Pi seeks truth in every situation. He finds truth in the details, his actual physical surroundings. Using a spiritual inclination with an awareness of science and his physical surroundings, Pi is able to adapt to any situation. Pi is a realist and a spiritualist; in this way, Pi is a paradox. But in combining science and religion and in his ability to adapt to such different, often oppositional, circumstances, Pi makes this paradox work.