Just a simple question, which one has the closest meaning of soulmate, 運命の人 or 心の友? Are they interchangeable in Japanese?
運命の人 is used exclusively to refer to a romantic partner or (future) spouse with a lifelong bond with you. 心の友 is used to refer to a close friend, typically of the same sex, as the kanji suggests. They are not interchangeable at all in Japanese.
運命の人 is a relatively common phrase. On the other hand, 心の友 is rare except that a manga character ジャイアン uses this often. When I hear 心の友, I can't help recalling the character, and I believe many native Japanese speakers are like me. See image search results for 心の友. Some online glossaries define this as a ジャイアン語 ("Jaian-ese") term.