Eva Merthen was born in Turku in 1723, one of eight children of the merchant Carl Merthen. Her paternal grandfather and his father were merchants in Turku, the last-mentioned apparently coming from Lübeck. Eva Merthen was dark-haired and of a stately beauty - she had especially beautiful eyes. All of the sources telling of Eva Merthen, including those published during her lifetime, state that she had children by Keith, but it has not been possible to identify them.
She was in relationship with the General James Keith during the Russian occupation of the Swedish province of Finland during the Russo-Swedish War (1741–1743).
Her father was merchant who came from Lübeck. Carl Merthen became a magistrate in 1728 and chief judge of the town's administrative court in 1732. He represented Turku at four sessions of the Diet between 1723 and 1741 and was later regarded as a member of the 'Hats' faction. During the Russian occupation in 1742/43, he was among the leaders of the town and was appointed to the Russian-period Imperial Court of Appeal as a permanent member of the college of judges. Carl Merthen died on 31 March 1743, before the end of the occupation.
