who helped siddhartha the most in findig inner peace? why?

There are a few people that help guide him. I'll go with the character of Vasuveda. Vasuveda is the ferryman with whom Siddhartha lives for the last third of the novel. He is a reticent and uneducated man, but he is an excellent listener and teaches Siddhartha through example. Most notably, Vasuveda teaches Siddhartha to listen to the wisdom of the river, a wisdom which leads both Vasuveda and Siddhartha to enlightenment.