Bob Salmon

 was a computer programmer and a hacker.

The Fifth Doctor got his help in 1975 to stop Professor Xerxes from installing a backdoor to navy software that would have allowed access any time he wished. In 1980, the Sixth Doctor asked him to aid in the retrieving of the cargo of an Eridani spacecraft that had crashed on Earth years before.

When they first met, the Doctor told Bob about regeneration. When they saw each other again five years later, the Doctor thought Bob's reaction was to his physical change. However, Bob was more shocked by his change in attire. (PROSE: Blue Box)