Um… what about the term “soulmate”?
I was under the impression that “soulmate” always has romantic connotations.
The other thing about soulmate is that it implies that there is only one person in the entire world that you can have this kind of understanding with, whereas drift compatible does not have that baggage.
Hm, I didn’t see it as strictly romantical, the exclusiveness is a point, though… I’m curious if there’s a word in another language that conveys the meaning of a non-romantical non-exclusive deep bond between two people. Do you know of any?