Thin
You are so sick that you've come to look like the humble stool: thin. Eerily thin, in fact.
Imagine standing behind a stool and being able to disappear behind it completely. That's how thin this sickness has made you.
そう
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None
When you're this thin, your soul (そう) is visible from outside of your body too.
Picture your soul pulsing through your thin body. You have no body fat to cover it up, so everyone can see it pulsing and lurking in there. This is why you're hiding behind one of the humble stools in your house.