Sally Solomon (played by Kristen Johnston) is a highly trained military officer and weapons expert. Her role is that of head of security where the mission is concerned and also second-in-command to Dick. She is frequently referred to as Lieutenant by the other members of the unit suggesting her rank.
In this interview, Kristen Johnston describes how she found playing the character of Sally and her experience of being in 3rd Rock:
For the purpose of the mission to Earth - and as part of the deception - she plays the role of sister to Dick and Harry as well as Tommy's aunt.
Her primary objective is to maintain the safety and security of the unit, something she takes very seriously and adheres to rigorously. Her instructions prevent her from killing humans unless they, by accident or otherwise, discover the truth about their identities or otherwise jeopardise the mission.
When the crew arrive on Earth she is also assigned the role of being 'the woman', apparently because she 'lost the toss'. This causes her no end of frustration as she is clearly the most outwardly macho of the group. Being 'the woman', she takes it upon herself to do all the housework, since that appears to be the Earth custom! A bit like Dick's teaching role, she pretty much sucks at it but persists for the sake of the mission and in order to 'blend in.'
From what we are able to discern, the aliens come from a culture that is androgynous or asexual. In the episode where Sally walks into a gay bar and is mistaken for a transvestite, she reflects afterwards on the experience by remarking: 'remember when we first got here how we found the notion of two genders to be limiting?'
Gradually Sally warms to the role or being female, discovering some of the more interesting aspects of womanhood along the way. She can take solace in the fact that she is experiencing something about the human condition that the other three cannot even fathom. They closest they come to understanding this is a frequently expressed desire to 'touch her boobs' - something which she expressly and instinctively forbids them to do.
Her choice of close companionship reflects her military bearing and training, but also bad judgement. Although she dates several eligible types, she can't resist the lure of a man in uniform and finds true love with local policeman, Officer Don - who suffice it to say is beneath her in practically every department and not just physically.
In the end, the alien Sally's clearly expressed penchant for violence morphs with her human form to produce a fiery, angry and truly scary Amazonian type, yet not entirely bereft of feminine sweetness and charm. Being human helps Sally to realise that she doesn't need to rely entirely on violence to get her own way - it also helps to be stunningly beautiful and have a great body.