I'm not entirely sure if the term "kindred spirit" was coined by
L. M. Montgomery in her
Anne of Green Gables books, but that's where I first encountered it. My definition actually differs slightly but significantly from the
Anne definition.
A kindred spirit is a rare kind of friend. Rarer still than the oft-spoken-of "best friend", I would venture. A kindred spirit understands you better than you understand yourself, and you return the favor. Kindred spirits are hard to come by and harder to keep. Probably in some ideal universe, kindred spirits make good romantic companions, but it would be tricky.
One of the lessons of Strangers in Paradise is that "Platonic" friendships are just as fragile and complicated as their romantic counterparts, and the kindred friendship is no exception. If you run across one, count yourself a lucky person.