Nyssa

Nyssa of Traken, or simply Nyssa, was a native of the planet Traken, the homeworld of the Traken Union and daughter of the respected Consul Tremas. When Tremas was killed, his body taken over by the Master, and she had nowhere to go, so she traveled with the fourth and then the fifth incarnations of the Doctor.

Life on Traken
The daughter of Councillor Tremas and stepdaughter of Kassia, Nyssa was a child of privilege on Traken, the center of the Traken Union. She was gifted with a brilliant intellect, an expertise in bioelectronics, and a gentle, sensitive and compassionate soul. Nyssa first met the Doctor and Adric as they defeated the Master, who had already used and killed Nyssa's step-mother, Kassia. The Doctor and Adric defeated him, but subsequently, her father's body was usurped by the Master's life force and taken over. Subsequently, the Master left Traken. (DW: The Keeper of Traken)

Travels
Not long after, the Watcher removed Nyssa via the Doctor's TARDIS to the planet Logopolis. There, she witnessed her planet being destroyed by a wave of entropy. She was also present, with Adric and Tegan Jovanka, a Human from Earth, when the Doctor regenerated into his fifth incarnation. (DW: Logopolis)

With Tegan, she gave crucial assistance to the the newly-regenerated Doctor, effectively representing his scientific side while he went through his troubled regeneration. (DW: Castrovalva) Having no home any more, Nyssa joined the crew of the TARDIS. Following an adventure with the Monarch, she fainted (DW: Four to Doomsday) on account of stress and spent a few days in recovery. (DW: Kinda)

The death of Adric (DW: Earthshock) was a severe blow to Nyssa, as it was to Tegan. Tegan herself vanished, left behind due to an oversight of the Doctor's. (DW: Time-Flight) After this, Nyssa and the Doctor traveled together. Nyssa coped with her newly awakened telepathic powers (BFA: Winter for the Adept, Primeval) and also witnessed the birth of the Cybermen on Mondas. (BFA: Spare Parts)

By chance, the Doctor and Nyssa met Tegan again in Amsterdam. (DW: Arc of Infinity) Nyssa faced the challenges of her adventures with the Doctor with both determination and the serenity of her Trakenite birthright. She turned her sorrow in being the last of her people into a desire to help others. It was appropriate, therefore, that she left the TARDIS to help the crew of Terminus to find a permanent cure for Lazar's Disease. (DW: Terminus)

After her travels
She would she eventually leave Terminus and settled down as an academic in a university on an unspecified planet. (PDA: Asylum) Eventually, she married and had children with a man named Lasarti, who studied dreams. During this later period of her life, she reunited with the Doctor and again assisted with his regeneration into his next incarnation. (BFA: Circular Time)

Personality
Apart from the Doctor, Nyssa had more maturity than her fellow travelers. She did not brag about her scientific knowledge or believe that it made her superior to others.

Tegan noted that Nyssa preferred to "care and share", which Tegan sometimes found intrusive and not her way. (MA: Goth Opera) Nyssa was often the peacekeeper among the crew of the TARDIS, though she often found Adric irritating at times. She had probably the closest relationship with this incarnation of the Doctor of any of his companions. Nyssa's nature made it a bit hard at times for her to relax and enjoy herself.

She tended towards pacifism, though on one occasion she destroyed a Terileptil android (DW: The Visitation) and attempted to use the threat of violence in order to save the Doctor's life. (DW: Arc of Infinity) Even though her world and her people are all gone, she always tried to live by the principles it stood for when she traveled with the Doctor.

She was left with a profound desire to put her skills to good use helping others in need, something she proved by staying aboard Terminus risking her life.

Behind the Scenes

 * Nyssa was originally meant to have appeared only in The Keeper of Traken as a supporting character. She was therefore the sole creation of writer Johnny Byrne, to whom royalties must be paid when the character is used.  She is the only series regular to whom the BBC does not own sole copyright.
 * Peter Davison was known to have preferred Nyssa over any of his other companions. He intervened on several occasions when John Nathan-Turner attempted to write the character out of the series.
 * Perhaps in deference to Davison's preference for Nyssa, Big Finish greatly expanded Nyssa's role. They have suggested that gap between Tegan's departure in Time-Flight and her return in episode 2 of Arc of Infinity was very much greater than it appeared on television.  Starting with The Land of the Dead, the Fifth Doctor began a long series of audio adventures with Nyssa as his sole companion.
 * Nyssa's original costume presumably originated on Traken, and suggested her high-born status. It consisted of a velvet jacket, a delicate-looking skirt, and an ornamental hair comb. This look persisted through Logopolis.  However, as she evolved from guest to regular companion, it became apparent that the actor would need more practical clothing.   Therefore, she lost elements of her original costume in Castrovalva, most notably exchanging her skirt for a pair of trousers she found in the TARDIS. According to production notes on the DVD release of the preceding story, Logopolis, the second iteration of Nyssa's "royal Traken" look was inspired by a rehearsal in which actress Sarah Sutton wore Nyssa's top, but her own corduroy pants.  She completely abandoned the "royal Traken" look in Snakedance, and thereafter changed her look every story until her departure in Terminus.

Key Life Events

 * Nyssa meets Adric and the Doctor on Traken. (DW: The Keeper of Traken)
 * Nyssa joins the crew of the Doctor's TARDIS. Entropy destroys Traken. The Doctor regenerates into a new form (DW: Logopolis)
 * She witnesses Adric's death. (DW: Earthshock)
 * Nyssa decides to stay behind on Terminus. (DW: Terminus)
 * Long after the events of Terminus, Nyssa meets a younger version of the Fourth Doctor than she had previously encountered in The Keeper of Traken. (PDA: Asylum)
 * At a point that may or may not be after the events of Asylum, Nyssa marries Lasarti and has children by him. (Mentioned BFA: Circular Time)