I gravitate towards a great many introverted NFs in my life. There is one lady who has that extremely mellow demeanor and cares a great deal for animals, expressing concern for their plight with humans. I thought for certain she was an INFP or ISFP until I found out that she is extremely systematic and prefers structure and steps in learning. I also have some confusion because my sister and myself are both strongly NF, but between the two of us, it is like a swirly cone of the 8 functions. For years I was certain she was an INFP and I'm an INFJ, but as we grow older, the reverse case could be made. She is much better at expecting a high standard of behavior from her students and has been able to make friends and has more of a need to participate in a church and other socially connecting activities. In some ways she is better with Fe than I am, but she keeps her environment more random and mine is more ordered. To outsiders I can appear more J-like, but I have almost no ability to enforce behaviors on other people, and I'm continually pressured to do this at my work. And yet my sister has more of the focused passion of Fi than I do when she cares about issues. Also, I experience both Ni and Ne, and I think my sister does as well. She can create complex, imaginative stories with histories of imaginary societies interacting, similar to Tolkien. And I get tons of weird, abstract impressions after people and animals die, or when I see a stranger I can rapidly see their face move through every emotion, or I get crazy memories of past lives. I think my sister gets impressions as well. When I read Jung's descriptions of Fi and Fe, I find both somewhat alien. I find the Fi description more harsh, but also find the Fe more alien. I had to laugh when Jung writes that the Fe woman is concerned with marrying an "appropriate" partner. My romantic choices are always completely at odds with what society would presume for me.
I'm also dating someone whom I cannot figure out if he is ENFP or INFJ, and I realize that in its textbook form it should be obvious, but when applied to actual people, they can display all 8 functions rather clearly. Externally people would tend to see him as ENFP and me as INFJ, but in our actual relationship it comes across more like he is INFJ and I'm an INFP. It's not clear at all in many cases.
It tends to be easiest to type acquaintances because you can see enough to make patterns, but also little enough to feel comfortable with stereotyping the person.