expression, noun
kindred soul; congenial friend
A somewhat literary or formal expression for a close friend with whom one shares a deep mutual understanding.
彼は私にとって会心の友だ。
He is a kindred soul to me.
会心の友と呼べる人はそう多くない。
There aren't many people you can call a congenial friend.
心の友 is similar in meaning but more common and slightly less formal; 会心の友 carries a nuance of mutual understanding and shared feelings.
From 会心 (かいしん, 'mutual understanding') + の + 友 (とも, 'friend'). The exact historical derivation is uncertain, but the phrase is conventionally associated with the idea of a friend who truly understands one's heart.