Karen Rhodes was a member of the Ares I crew and therefore one of the first 6 people to step foot on Mars. She is first mentioned in Diary Of An AssCan: A Mark Watney Short Story and this is one of the only name mentions of other astronauts from the Ares Program.
According to Mark (whom she is apparently friends with, if not an acquaintance of), they got a parade when arriving back from Mars. She is the Mechanical Engineer in the first crew of the Ares Program. She hates the Missed Orbit Scenario plan as much as Mark does, if not more, and he texted/called her about it.