To the (hopefully few) people who were angsting about “soulmates” as a translation, there’s a great old Eikichi Yazawa tune called “Shinyuu” that’s about two male, bousouzoku “shinyuu” and things ending because one met a woman and it is a BREAKUP SONG. It’s heartbreaking, and beautiful (aside some of the mid-key misogyny because 1970s thug life social constructs, see also Shonan Junai Gumi).
It’s a bit more than “BFF”. We don’t have a word.
We have the same thing happen, of course.
My friend of approaching 30 years and I cannot remember who said what in conversations we can remember clearly, because we were basically the same person. We spoke so similarly that old recordings (we were a musical duo, so we have recordings of chatter) we speak exactly the same. You have to go by pitch and timbre because the inflection, phrasing, vocabulary etc were all the same. We still speak fairly similarly.