Earlier quoted context omitted.
Coming from Spain we have a similar difference between like and love that you don't in English so it might be difficult to grasp. Basically we have a middle-level between both of them: 1. I like you. Me gustas. Suki-desu -> very early in the relationship, or to start off 2. [blank]. Te quiero. Daisuki-desu -> formal couple, to express feelings of love 2.5 I love you -> formal couple after long time dating I believe,…
There's also "koi shiteru" which is even higher maybe "you're my soulmate"
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.