Professor Willow
Like all Pokemon Games, the first non-playable character (NPC) your trainer will be introduced to in the Pokemon GO Universe is the Professor. In Pokemon GO, the Professor is named Willow, and, like all Professor's before him, he will get your journey started. He will get to know a little about yourself, whether you're a boy or girl and what you'd like to be called. He'll even get you started by helping you find and catch your Starter Pokemon.
Like a few other Professors, Willow will also play a role in assisting your trainer with a few tips on how to begin your journey. As you are traveling, you may even get a tip here and there from him with some information (like Pokemon Gyms, if you are not yet at Level 5 and haven't joined your team). You can even access all of his tips by clicking on the Poke Ball on the main screen and then clicking on the Tips Icon in the top right corner of the following screen.
He doesn't play that extensive of a role, he's not really even that important of a character outside of gifting you with your first set of Poke Balls and helping you choose what team you will join at Level 5, but Willow plays a key part in the beginning of Pokemon GO, especially if a trainer is new to the way of Pokemon as a whole.