Doc's a good example of a bad first impression. Wanda first sees Doc as someone who is going to cut her open to see how she works. This is… only half true.
When Wanda first sees him, she says, "The doctor eyed me with a frank and almost friendly curiosity that made me feel cold deep inside my bones" (20.100). She interprets this frank curiosity as a psychotic desire to kill her. Although he's operated on body-snatched humans before, he's done it out of a need to find a "cure." Like many people, Doc will do whatever it takes to save his species.
But without Wanda, it's not enough.
The aliens' healing medicines are the best thing that's happened to healthcare since penicillin. As Wanda describes them, "They heal, rather than merely treating symptoms. No trial and error. Of course the human medicines were discarded" (21.37). By learning to use the medicines Heal, Seal, Smooth, and more, Doc blends human compassion with alien technology and becomes one of the most effective doctors the Earth has ever seen. (Sorry, Dr. Oz.)
As Doc gets to know Wanda for the "person" she is, Wanda starts to see his compassionate side. It begins when Walter, dying of cancer, believes Wanda to be his dead wife, Gladys. Wanda plays along and provides the kind of relief for Walter that medication can't. Doc says, "She's the best painkiller I've been able to find" (31.177). (Better than morphine?)
Wanda's compassion helps Doc learn to trust her. He tells Wanda his real name (Eustace), and Wanda says, "It was strange to see such depths of compassion in a human. [...] The man simply could not be a torturer" (31.156). Even when she sees that he's operated on a soul, killing it in the process, she's able to forgive him. After an initial emotional reaction, she decides to show him how to remove a soul properly. The two of them work together for the benefit of both races.