Which character is secretly married when he tries to elope with Natasha?

The character secretly married when he tries to elope with Natasha is Anatole Kuragin.

Anatole is Prince Vasily Kuragin's reckless, pleasure-seeking son and Hélène Kuragina's brother. He meets Natasha during her visit to the Moscow opera, while she is separated from her fiancé, Prince Andrei, who is away in Europe during the year-long delay imposed on their marriage.

Anatole courts Natasha intensely and persuades her that he loves her. He arranges an attempted elopement with help from his friend Dolokhov, but Natasha's cousin Sonya discovers the plan and prevents it. Pierre Bezukhov then reveals the crucial fact: Anatole is already married to an unnamed Polish woman whom he had abandoned.

This episode is often simplified as an ordinary love triangle, but Anatole's existing marriage is what makes the elopement especially deceptive. Natasha breaks off her engagement to Andrei before learning the full truth, and the scandal leaves her emotionally devastated. Anatole later loses a leg after being wounded at Borodino, where he is brought near the badly injured Andrei.