Gabrielle d'Estrées was born in 1573 and became mistress to Henri IV at an age of around 18 in 1591. She persuaded Henri to renounce Protestantism and become Catholic, ending religious warfare. Gabrielle worked the Catholic and Hugenot sides to broker peace. Henri arranged to have Cabrielle's marriage annulled and also had his marriage to Marguerite of Valois annulled. In 1599, she died in childbirth before the pair could be married. Her Wikipedia article is here.