Hathaway is the doctor and geologist from the Fourth Expedition. He decided to settle on Mars with his family—his wife (Alice), two daughters (Marguerite and Susan), and son (John)—to do "remote research on Martians." So, right away we know that Hathaway is one of the good guys, because he's curious and open about their new home.
Because Hathaway and his family were doing remote research, they never made it to the rockets to go back home to Earth. Hathaway's family then all died of an unknown disease. (Kind of like the Martians, who all died from chickenpox brought by the colonizers from Earth.)
To ease his loneliness, Hathaway built himself robot versions of his family. They've helped keep him sane for the past twenty years. Even though Hathaway doesn't occupy a lot of space in the book, he thematically connects several stories.
In a way, Hathaway is an interesting contrast to both Walter Gripp and to Stendahl. Hathaway is a tragic "last man," while Gripp is a comic "last man." (See "Tone" for more about comedy and tragedy.) Hathaway uses robots for good (or at least neutral) ends, while Stendahl uses them to kill. And like many of the characters, he's an example of the extremes people will go to when their lives are disrupted.