https://www.gamespot.com/articles/metal-gear-solid-5-update-lets-you-play-as-quiet/1100-6460705/

A new update lets you play as Quiet in Metal Gear Solid 5: The Phantom Pain's Forward Operating Base missions. The update adds several other changes as well, including new online development items and another difficulty for Event Forward Operating Base missions.

Quiet is a sniper who loses to Venom Snake in a duel in Afghanistan. She becomes a prisoner at Mother Base until she proves she can be trusted to work with Diamond Dog personnel. She's then available as a companion character who can assist the player on missions.