Nyssa is the daughter of Tremas, First Scientist and Consul of the Empire of Traken. Tremas is now dead at the hand of the Master, who has commandeered his cadaver by way of a thirteenth regeneration. She is eighteen, of noble birth; an attractive young girl with values and skills deeply rooted in her Traken past. Tutored by her father in the advanced sciences, she is already a skilled apprentice in bioelectronics, a discipline in which her people excelled.
Nyssa is an open young woman. Idealistic and pragmatic by turn, she has an abiding belief in the essential goodness of all things, which sometimes blinds her to the less overt manifestations of evil. For example, in meeting the Master for the first time in his new guise she has mistaken him for her father, only being rescued from his clutches by the intervention of Adric and the Doctor. A threat once visible, however, brings out all that is best in Nyssa: calm assessment, lightning judgment, and nicely judged action.
Nyssa's aristocratic background sometimes leaves her oblivious to the simpler needs of others, and occasionally prevents her from seeing the funny side of situations. Adric, an orphan like her, is very fond of Nyssa, but at times her innocence, seriousness and inability to compromise seem to him like deliberate stubbornness.
The Doctor feels, irrationally, a sense of responsibility for the death of her father, but has too much respect for her individuality to see himself as any kind of substitute. He appears, in his offhand way, to en]joy having her around and being in some small part a force in her spiritual development. He would never allow it to be seen that deep, deep down inside the presence of all these young people in the TARDIS is very wearing!
BBC Character Outline. Johnny Byrne, John NathanTurner, Christopher H. Bidmead. 30 Oct 1980.