Ok, I agree up front that my 18-year-old step-daughter is more than just "an interesting problem" for the MBTI. She's also my step-daughter since 2006. She took the MBTI in high school and scored ISFP. But this is dead wrong because she's one of the most extroverted people I've ever known. ESFP fits her much better.
Since I've arrived onto this forum I've learned more about tertiaries and function analysis than I ever dreamed possible. I've been forced to look into Jung's work on psychological types again. So now, when I consider my step-daughter's type, I see that the Fi in ESFP is entirely lacking (undeveloped) with her. And if I had to consider her cognitive type properly, I would have to say it's more like SeFe, for which there is no single MBTI description. In Jungian terms, her main cognitive functions are "undifferentiated," and she has no secondary or "helper" function.
At least there is no internal conflict in being SeFe that I've seen. Perhaps it would be better if there was. But it does indicate some more or less serious problems, ADHD and Bipolar disorder for starters, perhaps because there is no Fi to slow her down, to cause her to stop and ponder her next move before making it, leading to extreme impulsiveness when she's not on her meds.
I don't know if there's any kind of therapeutic care possible here beyond chemical, or if it's possible to develop the Fi that should usually go with being an ESFP.