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nature (of a person); disposition; temperament
nature (of something); character; kind; sort
「質」 has other readings with different meanings.
彼は非常に真面目な質の人ですね。
He is a person of a serious nature.
この水は鉄分が多い質で、紅茶には向かない。
This water is the kind rich in iron and not suitable for brewing black tea.
生まれつきの質を変えることは難しいです。
It is difficult to change one's innate disposition.