Although Pulcheria Alexandrovna was forty-three, her face still retained traces of her former beauty; she looked much younger than her age, indeed, which is almost always the case with women who retain serenity of spirit, sensitiveness and pure sincere warmth of heart to old age.
There are no more uses of "serene" in the book.
Show samples from other sources
While I lie awake worrying, she sleeps serenely.
She remained serene even when he was yelling at her.