Glamorous young female intelligence agent newly attached to UNIT. Keen, professional, lots of charm. Works with the Doctor. Needs to be involved with the story in an active way, not just as a screaming heroine or passing the Doctor's test tubes. Not a scientist, though with enough basic background to know what's going on.
That was the original outline for the character. Several highly attractive, glamorous actresses were auditioned for the role, though Barry Letts and Terrance Dicks found the bubbly, scatter-brained, short-sighted Katy Manning more endearing than the more straightforward alternatives.
Katy remembers being asked in her audition to initially be frightened of what her character thought was a monster, then to laugh in relief when she finds out it isn't. She has said that since this was very close to her own state of emotions at the time that she didn't have any problem auditioning that way.
Jo was a new recruit to UNIT who only got the job because her uncle had pulled strings for her. The Brigadier, not sure what to do about her, handed her over to the Doctor, pointing out that all he really needed was someone to hand him his test tubes and tell him how wonderful he is.
Jo, with her short skirts and many rings, was clearly a product of the early 70's. At the same time, she reflected a return to the more traditional early Companion after the failed experiment of Liz Shaw, who while a strong and worthy character did not play off the Doctor in the traditional way. Jo screamed, got into trouble, and got captured, the time-honored role of all good Companions. She was supposedly trained in espionage and escapology, though these characteristics only seemed to surface when the plot required them.
Like all successful Companions, the character draws upon the actress. Jo, while intelligent and resourceful, was somewhat accident-prone, definitely scatter-brained, and tended to act first and think later. It made her an excellent foil for Jon Pertwee's strong, action-hero Doctor, and it is no wonder that Jo's departure is one of the most poignant in the entire series.
Back to the TARDIS.