Charlie Sato

Charlie Sato was a UNIT Warrant Officer who worked in the Vault.

Early life
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 right decision to save him. (AUDIO: Mastermind)

The Vault
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)

The Master's escape
Matheson and Sato were in the vault when 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 people's bodies. The Master later tried to use this event to manipulate him to allow him to escape. The Master managed to 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)

A return to duty
Charlie resumed his duties for the Vault. One of his missions sent him to Puerto Rico in pursuit of a piece of alien technology which was stolen by alien parasites. He was rescued from torture by the Eighth Doctor, who revealed that the object he was pursuing was a broken sonic screwdriver which was left behind and forgotten by one of his future incarnations. Charlie and the Doctor thwarted the aliens and recovered the screwdriver, although the Doctor insisted on keeping it for himself. (AUDIO: The Turn of the Screw)