Fi, or introverted Feeling, is dominant for IxFP, secondary for ExFP, tertiary for IxTJ and inferior for ExTJ. Unlike Fe, Fi leads you to draw ethics purely from an internal, subjective source and finds Fe's collective approach to morality shallow and fake. Since ethics are purely a personal ideal in Fi's view, all personal feelings are sacred and allowing any outside views to affect them is patently unethical. Fi treats ethics in the same way Ti treats logic, in that it's something that requires no external context to understand and that should not be influenced or changed by any outside forces.
The ability to express one's personal feelings and inner self freely and maintain a strong sense of personal uniqueness and individuality is of utmost importance to Fi. (It's also important to Ti, but for different reasons and in different contexts.) I have found that many Fi users dislike typology in general because they feel that "putting people into boxes" suppresses their sense of personal identity, and that people are too unique to be categorized so easily.
Fi people are typically very good at picking up emotional vibes in the tone of voice and word choice of others. They often know what you're feeling even better than you do because they're aware of the subtle effects that different emotional states have on our behaviors and can pick up cues about how you're feeling that you didn't even intend to give off. For this reason strong Fi users are profoundly empathetic and tend to understand and identify with basic human needs on a profound level. If you were to criticize someone's behavior from an Fi standpoint, it would be in the form of, "What you are doing is hurting my feelings/violating my ethics/preventing me from fulfilling my basic human needs." Strong Fi users often feel a certain connection to the beauty of nature, animals, and life itself, simply "knowing" deep inside themselves that life is sacred and all individuals possess inherent value.
A mature Fi user is extremely in tune with the emotional needs of others and very supportive of and responsive to them. An immature Fi user is overly preoccupied with his own emotional needs and will act passive aggressively toward people who don't bend over backwards to cater to how he feels. Note that both Fe and Fi users often feel a strong sense of moral obligation to their loved ones; the difference is simply the source of this obligation. If it comes from an external/objective cultural standard, it's probably Fe--if it comes from a personal sense of moral responsibility that deliberately blocks out external influence, it's probably Fi.
Fi doms are ethical perfectionists in the same way Ti doms are logical perfectionists. They seek a sense of internal balance and harmony with their surroundings that feels right in their own individual way. Note that introverted judgment (Ti/Fi) seeks depth and specificity while extroverted judgment (Te/Fe) seeks broad applicability. Ti wants to define exactly what is logically correct under an extremely specific set of circumstances that may never happen in the real world, while Te seeks widely applicable objective consensus that can apply in many different situations. Fi seeks to determine precisely what the user feels is morally right regardless of external application, while Fe seeks widely applicable, generalized ethical rules that can serve to govern entire groups. Ti/Fe = I think/We feel; Fi/Te = I feel/We think.