Senna is Enzo's favorite driver, who died tragically and mysteriously during a race in 1995. Enzo says that Senna was charismatic and daring, and Senna is often conflated with Denny. Both Senna and Denny are exceptional drivers in the rain, which speaks to their intellectual capabilities and unflappable attitudes. Despite this conflation with Denny, however, Enzo remarks that he himself is often more like Senna than Denny is, as Enzo wants to be more spontaneous and carefree than Denny often is willing to be. Senna provides Enzo with something to idolize and look up to and is a huge player in Enzo's conception of spirituality, as he acts like a positive counterpoint for the evil of the zebra. Senna's persona also informs Enzo's idea of what he'd like to be when he's finally human.
Ayrton Senna Quotes in The Art of Racing in the Rain
After the 1993 Grand Prix, the best thing I've ever seen on TV is a documentary that explained everything to me, made it all clear, told the whole truth: when a dog is finished living his lifetimes as a dog, his next incarnation will be as a man.
He died that day because his body had served its purpose. His soul had done what it came to do, learned what it came to learn, and then was free to leave. And I knew, as Denny sped me toward the doctor who would fix me, that if I had already accomplished what I set out to accomplish here on earth, if I had already learned what I was meant to learn, I would have left the curb one second later than I had, and I would have been killed instantly by that car.