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In Japan you never use such names of affection the way americans do, for example.
Kokoro is a word that connects the mind, heart, and soul into one word.
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Ken has also mentioned in his videos that his students are able to see everything he has to offer.
Whether you're going to be writing in hiragana or kanji, you'll need to learn how to write properly, as writing with the wrong stroke order can make it look like your sentence says something else entirely.