I just wanted to get some reasons why you like being an ISFP rather than anything else.
Novel characters exist to get into problems and wrestling themselves out of it. I've got some great ESTJ main characters and villains, a good ESFJ villain, a mediocre ENFJ manipulator, a nice ISFJ sidekick...
and some boring ISFP backdrop characters. The main reason why I'm on this forum is my eternal quest to give my characters more depth and different personalities. So again, ISFPs, unless you want to stay a backdrop, tell me how I can get you out of it!
Ha! I have a finished first draft novel in a drawer and several other writing projects ongoing now, fiction and nonfiction, plus experience with a critique group. I never used MBTI as a writing aid for creating characters, but it sounds effective.
I'll help.
Your ISFPs are full of
surprises.
One of them plays or is learning a very unusual instrument. Another expresses creativity in small, unconventionally humorous ways during the day -- in dress, in a packed lunch, in gifts, in jokes. (Never using mean humor, though.)
An ISFP in your novel may be extremely easy to talk with, the best listener you ever met, so other characters are going to confide in that character. Maybe one of those other characters will even work through a major problem and find release -- grow -- just from finally, finally, in the ISFP's presence, being really heard and accepted.
And speaking of finding release, well, there's the sensuality aspect to play up, if it suits your plot.
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an aside:
seen online in various places (I still have to double-check this one when writing, they overlap anyway I guess so I never quite know if I got it right):
sensuous/sensuousness vs. sensual/sensuality
sensuous implies gratification of the senses for the sake of aesthetic pleasure <the sensuous delights of great music>. sensual tends to imply the gratification of the senses or the indulgence of the physical appetites as ends in themselves <a life devoted to sensual pleasures>
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Anyway, what I meant above was, there's the (very possibly) good-in-bed part to play up -- or enthusiastic, positive, in-the-moment focus, which can translate to good-in-bed -- if it suits your plot.
Your ISFPs may also be quietly stepping back and seeing a bigger picture than some. One may end up as leader in some unexpected way simply because of that trait; think of the good supervisors who aren't getting all wrapped up in OMG thinking and negativity but calmly addressing the needs of the situation.
Make one of them involved with herbs or essential oils. Throw in an unusual pet, beautiful in its own way. Make one of them spiritual if you want, perhaps unconventionally so, but it's not a surface thing with the character. For example, if you make an ISFP character a Wiccan or a self-identifying pantheist, or anything that suits the time and place of the story, do some of that ever-engrossing internet research and get the useful, nurturing, freeing aspects of that worldview into your novel.
If you need conflict with the character, try having someone attempt to stomp on the ISFP's basic human rights or try to change them, or more subtly, have someone expect that the ISFP's way of looking at things must match theirs.
Good luck with your writing endeavors and have fun!