No, it is not who is playing Pet. IMDB has some additions that may be noteworthy to you, especially if you are really familiar with the novel.
Lee Hardee will play Aaron. According to The Host Wiki, he is a human of the cave, and “when the Seeker is brought to the cave he guards her along with Brandt.” This is a small role unless it was expanded for the film. Lee Hardee’s twitter account is @IAmLeeHardee.
There are many, many Seekers and Healers on the IMDB list as well. Most of these are not directly from the novel, so it looks like there are a lot more characters in the film than in the book. We will have to wait until the film to see how much screen time each character gets.