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one's nature; one's feelings; oneself
plain; unadorned; undecorated; unadulterated; au naturel
mere; poor
exceedingly
「素」 has other readings with different meanings.
彼女は素のままが一番かわいい。
She is cutest when she is natural.
彼女は素の自分を見せるのが苦手だ。
She has a hard time showing her true self.
人前では素の自分をなかなか見せたくない。
I do not want to show my true self.