First off, a quick (albeit not entirely serious!) character synopsis: (Taken from Doctor Who - Companions)

Name: Tegan Jovanka.

Occupation: Trainee air stewardess (with Air Australia).

Origin: Earth, Australia, 1980's.

Likes: Whining, whingeing, moaning, complaining... need I go on?

Dislikes: Almost everything.

Favourite swearword: Rabbits.

Joined the Doctor because: She mistook the TARDIS for a real police box (In 1981? Come on, even Tegan must have realised it was twenty years out of date).

Left the Doctor because: She had enough of all the murder and mayhem that tends to go with being a companion of the Doctor.

Companion most likely to: Be Prime Minister.

General description: Gorgeous brunette with a huge attitude problem. A request for a drink from this stewardess would probably have been answered with a smack in the mouth. She dressed to kill and then wondered why everyone around her died. Once described herself as a "mouth on legs".

Like we said, a bit tongue in cheek! But pretty accurate all the same. Tegan was meant to be a strong character and she certainly was that. Heres the character description JNT and Chris Bidmead came up with:

"Tegan is twenty one, an attractive and intelligent Australian trainee air stewardess, whose brash confidence in her own abilities actually conceals inner insecurity, a state of affairs that becomes clear in moments of stress.

On her way to her first real flight she accidentally blunders into the TARDIS and thus finds herself being inadvertantly abducted by the Doctor. Characteristically her inner bewilderment at the new situation in which she finds herself causes her to assume an attitude of overweening self-assertion, and she begins to take charge of the Doctor and Adric.

During the course of three stories Tegan's superficial self-assurance will build until it becomes a real problem for the other two occupants of the TARDIS, and it will need drastic action on the part of the Doctor to put things to rights and show her the error of her ways. She may or may not continue with the Doctor thereafter."

The actual course that the character finally took was thrown off tilt somewhat by the fact that a fourth member joined the TARDIS crew at that point in the form of Nyssa and less emphasis was put on the "drastic action" line of the story. However, Tegan did go on to stay with the Doctor for a total of nineteen stories.