Josephine "Jo" Jones (née Grant), known simply as Jo, was a civilian employee of UNIT. The Brigadier decided that Jo could serve as the Third Doctor's research assistant during his exile on Earth, replacing Liz Shaw, who had returned to Cambridge. Though the Doctor initially rejected Jo as scientifically incompetent, the Brigadier was correct in his prediction that what the Doctor needed was "someone to pass you your test tubes and tell you how brilliant you are". Jo and the Doctor quickly became almost inseparable.
She stayed with him for the latter portion of his Time Lord-imposed exile, and adventured with him for a short while thereafter. During her time with the Doctor, she arguably became more familiar with the Master than any other companion, and even learnt how to resist his direct attempts at hypnotism. She was also one of the first people to witness a meeting between different incarnations of the Doctor.
Jo's first meeting with the Third Doctor didn't bode well for the future, as she mistook an important experiment for an accidental fire. She ruined it by dousing it with a fire extinguisher, which earned her the Doctor's wrath. Despite this initial clumsiness, her failures fueled her enthusiasm, and she became more determined to prove people wrong about her. When, for example, she recovered from the Master's hypnosis, she disobeyed orders and followed the Doctor to Luigi Rossini's Circus. There, she proved her competence by rescuing the Doctor. Having been hypnotised by the Master once, Jo was able to resist his hypnosis on further encounters by reciting nursery rhymes.
Jo claimed to be a qualified agent, but she may have exaggerated about this, in the same way she exaggerated the extent of her A-Levels. After saying she took an A-Level in Science, she later revealed that "I didn't say I passed." She could ride a motorbike, use karate, pick locks, abseil, and even pilot a helicopter - all of which came in very useful during her time as the Doctor's assistant. These abilities combined with her enthusiasm proved Jo to be an asset to UNIT.
Jo did not initially believe the Doctor when he said he was a time traveller. For a substantial period of her time with him, she thought his tinkering with bits of TARDIS was pointless. In fact, she may have been with the Doctor for as much as a year before she even stepped foot into the TARDIS. she was extremely tentative during her first trip away from Earth. She repeatedly begged to be taken back home, before she realized that the Doctor had little ability to grant her wish. After the Doctor coaxed her, she did eventually step out of the TARDIS and into her first adventure on another world. Whilst Jo was certainly a member of the team, her individualism made her stand out in UNIT, and her desire to follow her own instincts and speak her mind brought her and the Doctor close. They were both very different from the norm in UNIT.
The duo were parted only when she became passionately involved in the growing ecology movement of the 1970s and got engaged to Professor Clifford Jones. They married soon after. She then spent the next four decades travelling the globe with her husband and her increasingly large family, until finally she reunited with a much later incarnation of the Doctor.