She originally met Vesemir in the winter of 1234 while he was in Oxenfurt for a contract and both noticed each other from afar. After having her governess deliver a letter to him, the two soon met and had a tryst before Vesemir had to run off as Mignole's father soon learned of it, leaving behind his gambeson in the process. While her father put out an arrest warrant for the witcher, he was never caught and Mignole kept the gambeson.
She later married but her husband died young, and thus the countess was left with a sizeable fortune that she used to collect witcher gear, though she held onto Vesemir's gambeson due to its sentimental value.
If Geralt talks to her before The Battle of Kaer Morhen: Geralt revealed Vesemir was still alive and doing well, though he and the other witchers never saw him with a partner for several decades. Mignole was happy to hear he was ok, though wondered if their fates would have been different if she had ridden after him. She then thanked the witcher for the information.
If Geralt talks to her after The Battle of Kaer Morhen: Geralt revealed the unfortunate news that Vesemir had died, but had died as a hero. While saddened by this, Mignole was thankful for the information.