I think Jayne is an ISTP, like Clint Eastwood or Steven McQueen. This type relates to the external world through extroverted sensory confidence (in these cases, as a gun-slinger) But their strongest personal characteristic is internalised logic. Note, that doesn't mean the person has to be very brainy, it just means they are unsentimental, focus on what is rather than what ought to be, and this is their strongest attribute. Their weakest attribute is feeling - they don't know how to express it or deal with it.
I think
This makes me think about what introversion is. Both extroversion and introversion are quite complex attributes, which can manifest in lots of different ways. I think introverts with dominant thinking (that would be ISTP and INTP) can be the toughest minded people of all because their priorities are influenced by neither emotion (F) nor sociability (E).
Incidentally