Are you sure this isn't actually "aishiteru (愛してる)"? I've never heard of 'koishiteru', but 'aishiteru' occupies roughly the same level you described.
Actually, I have heard of 'koishiteru' (which I assume is 恋してる) but usually in the euphemistic context of sex.
Additionally, and not totally related, "soulmate", rendered in katakana (ソールメイト, sorumeito) is often used to breezily describe someone, often a coworker or classmate, with whom you have some level of camaraderie or friendship with. It is a perfectly unserious concept, which is amusing given its weightiness in English.
