User blog comment:Benihime101/The Heart/@comment-1590714-20101127134041

The Japanese word "kokoro" can be translated as both heart and soul. In Ulquiorra's case, I personally prefer to think of it as the "soul". In Kaien's case, he probably means the bonds between people which are symbolized with the "heart", but again, it's not a very clear topic. I do not know whether or not we have any confirmation from one of the character books about the meaning of the word when referring to one of both cases, but what we can be sure about is that it is often used symbolically, meaning the spiritual bonds between people.