Dr. Deere is kind of a mystery. At first, it seems like he's only included in the game peripherally as Angela's fiancé, the plastic surgeon intern. Grace thinks it makes perfect sense to include him because soon he'll be part of the family, and her narcissistic idea keeps us from wondering about the reasons why Dr. Deere is included... at least at first.
We suggest two explanations for why Dr. Deere is part of the wild Westing game.
First: his plastic surgery training. Dr. Deere knows enough about plastic surgery to recognize when someone, like Otis, hasn't had it. Maybe, though, because he's still in training, he doesn't realize that Sandy's face is all kinds of unnaturally screwed up. He provides key evidence in the trial scene when he admits that the body he saw in the coffin might not have been a real one. (Jake, the other individual with medical training, wasn't there at the first will reading, so he couldn't have been used as a witness for this. Plus, don't forget, his wife insinuated that he's not a "real" doctor.)
Second: remember how Chris says everyone got set up with his or her perfect partner? That's perhaps most true in this instance—Dr. Deere is paired with Chris. Dr. Deere starts off as somewhat callous—he says he's not a caretaker, and he's not there to give medical advice, and he also keeps asking Chris to sign over the money. Yet, later in the novel, he seems to soften, and comes up with the idea to take Chris in to the hospital for a revolutionary new medicine that might help his condition.
He does this out of the kindness of his heart and seems to forget about the money that's at stake. Without him, Chris might never have gotten access to the medicine that allowed him to better interact with the world and go to college. So what he does here is a true, good deed.
Finally, while we don't have much background on his relationship with Angela, or why they got together in the first place, Dr. Deere shows patience, perseverance, and fidelity when she breaks up with him to go back to school. He's still waiting for her five years later, when she's begun to reclaim her identity as a person, and he's able to convince her to give their relationship one more shot. That's what we call a standup guy.