Dominant Si: Martha is constantly comparing the present to her past experiences and learning from them. She always asks the Doctor about the minute details of how the Tardis works and how such-and-such will affect history. She has a great memory and can babble details when needed. She has a particular affinity for recalling events in recent history, and always knows the political situation for the last several years. She notices when evidence proves something to be different than her prior perceptions of it, and knows when people have changed from what they used to be (in her mind). When she first meets Shakespeare, her first comment is on how different he is from what she was taught. Martha values tradition and tends to role with the laws of the land (voting for Saxon for instance).
Dominant Te: Martha is a take-charge type of person who willingly steps into leadership rules. She can set aside her emotions to make harsh decisions (like leaving the Doctor to travel the world and save him). Martha has a hard time accepting anything as truth until she can see viable, external evidence for it. For instance, when she first meets the Doctor, she takes a longer time than most of the other companions to accept the idea that he is an alien time-traveler. However, she readily accepts the fact that she is on the moon because the evidence is right in front of her, impossible to argue with. Martha doesn’t have a problem with deadlines –I mean, she travelled the world under the deadline of total extermination and still got back in time to save the Doctor on the right day– how efficient is that?
Tertiary Fi: Martha takes a really long time to every open up to the Doctor about how she feels about him, which consequently results in his never noticing her affections. This is furthered by the fact that she tends to keep her emotions under the surface where others can’t visibly see them. Martha is extremely loyal to those she loves and will compromise herself to help them. Martha decides to leave the Doctor completely of her own accord, without the input of her family or friends. Her sense of right and wrong is self-defined and extremely strong. Martha wants life to be meaningful and so she devotes large allotments of time to trying to better the world through whatever means she can.
Inferior Ne: Martha can always think of possibilities as to what could happen should she make a choice, and she always wants to know the answer to her questions. She consistently asks the Doctor questions regarding the rules of time… “What if we do this?” It’s always hypothetical what-ifs with Martha. She’s good at coming up with creative solutions so that her enemies won’t always detect what she’s doing and then uses Te to execute those solutions.
I’ve seen her typed as INTP. For a while, I thought, “yeah, maybe…” but eventually I settled for ISTJ, the deciding factor being her use of Te over Ti. She’s good at organizing people, but doesn’t really spend a lot of time pointlessly analyzing information –she wants to do something with that information.