Charlie Sato was a Warrant Officer who worked in the Vault.
As a child in San Francisco he experienced an earthquake which caused his house to fall down, killing his father as he went back to save his sister. After his father died he became a soldier to show his mother that his father made the decision to save him. (AUDIO: Mastermind)
When Charlie first became a warrant officer for the Vault in 2011, he was given a tour by Captain Ruth Matheson, and in the Vault they came across four objects: An army jacket, a painting, crystal and a wax cylinder, which bore a significance. Each was used in one of the Doctor's many adventures. (AUDIO: Tales from the Vault)
Matheson and Sato were in the vault when The Master awoke and went to visit him. He was the first into the part of the vault where he was kept and interviewed him about how he came to be in the Vault. He got disgusted by the Master taking over peoples bodies. The Master later tried to use this event to manipulate him to allow him to escape. The Master managed succeed in tricking him and Matheson to allow him to escape in his TARDIS. (AUDIO: Mastermind)
After the escape of the Master, Sato was held at UNIT's private residence for compromised UNIT soldiers. In 2014, Sato was brought back into active service by Mike Yates, who needed his help to investigate what had happened to the Vault since he and Matheson left. (AUDIO: The Screaming Skull)