Serena
Serena, like Romana, was a Time Lady, and is technically the first true Time Lady companion The Doctor ever had - Susan, according to all information, was not yet fully qualified at the Academy, so can't really count as a true Time Lady in the strictest sense of the word.
When Selena first met the Second Doctor, she was working in the Capitol Library, studying records of the presidential election as a means of learning more about Time Lord politics, possessing ambitions at a position in the High Council. Despite some of her obvious advantages in Time Lord society, such as being a member of an old and powerful family, exactly what it was that led to her being selected as The Doctor's companion is unknown at this time. Contacted by the Celestial Intervention Agency, she was filled in on The Doctor's recent history - including, naturally, his capture by the Time Lords after calling them for aid in ending the War Games - and then told that she would now be going with The Doctor as the representative of the Agency while he investigated recent changes in Earth's history.
At the first meeting between the two Time Lords, there was a certain hostility between The Doctor and Serena; The Doctor objected to having a companion forced on him like this, resenting Serena's claims that she would be his supervisor, while Serena was opposed to The Doctor calling her his assistant, and insisted that The Doctor was merely a convict on parole. Despite their initial anger at each other, The Doctor and Serena agreed to work together, departing for the earliest anomaly in a Type 97 TARDIS that the CIA had provided, since they felt The Doctor's own TARDIS was too obsolete for such work - this TARDIS even had a working chameleon circuit.
Despite their initial disagreements, however, it should be noted that, even before meeting The Doctor face-to-face, Serena felt that he'd been treated unfairly; upon learning that he'd only been captured because he wished to save the humans involved in the War Games, she commented that his original sentence of execution seemed an unfair reward for his altruism. As they continued to work together, Serena came to understand more about The Doctor's reasons for leaving Gallifrey, and began to see that, for all the charges put against him at his trial, he had only interfered because he believed it to be right, not because he simply felt like it, and had left Gallifrey mainly because there was no place on it for someone with his principals. She even seemed to come to care for The Doctor in her own way, apparently exhibiting jealousy of the Countess's fondness for the Time Lord. Despite her naiveté about the universe outside Gallifrey, Serena proved to be a competent and able companion for The Doctor, managing to escape a Player called Valour who was holding her hostage while The Doctor was forced to aid the Countess in constructing a nuclear-powered submarine that would allow Napoleon to defeat Nelson at Trafalgar.
However, Serena's time with The Doctor was, like the Sixth Doctor's companion Claire Aldwych, cut tragically short after only one adventure with him. Having learned that the Countess intended to assassinate the Duke of Wellington at a ball held on the eve of Waterloo, The Doctor and Serena attended the ball to find out what had happened, and Serena took the shot intended for Wellington herself; the musket ball, fired at her from practically point-blank range, destroyed both her hearts, thus preventing her from regenerating. She was buried in a simple grave, the only thing on her gravestone being SERENA, and, after the battle of Waterloo, it was visited by The Doctor and Wellington, who assured The Doctor that he would always remember the role he and Serena had played in the battle. Once back on Gallifrey, having unmasked a Time Lord who'd been collaborating with the Players, The Doctor refused to go on any more missions for the Agency until certain conditions were met, including an order that Serena's name be placed on the Gallifreyian Honour Roll and that he be allowed to tell her family how she had died.
Despite The Doctor's gratitude that Jamie would be restored to him for his next mission, it was evident that he still regretted Serena's death; indeed, when General Grant challenged her murderer to a duel, The Doctor was able to crush his conscience with a surprising amount of ease, and his promise to tell her family about their loss also says a great about what she came to mean to him (Although, admittedly, in any other case where companions have died, either he couldn't contact their families (Adric) or he didn't know who their families were (Claire)). (PDA: World Game)
