There's kind of a tradition among Doctor Who fans that of all the actors who have portrayed the Doctor there's one who is "your" Doctor. Usually it's the one you watched on TV when you were twelve. For me it's Tom Baker, and "my" companion is of course Romana II, as played by Lalla Ward. I have mentioned that I had a severe crush on her when I was a teenager. Who wouldn't, really? As Romana she was so cute and smart and calm and brave and confident. Even against at times stiff competition she's hung on to my "favorite companion" prize ever since. The terrible Doctor Who fanfiction I wrote as a freshman in college? It all stars her.
On the TV show, Romana was last seen abandoning the Doctor to hang out in a pocket universe called E-Space. However, in the various spinoff novels that started pouring out after the show's cancellation in 1989, the character eventually popped up again. I haven't read any of that storyline myself -- I largely dropped away from the New Adventures books as most of them got a bit too inside baseball for me -- but according to the novels, Romana came back from E-Space somehow, returned to Gallifrey, and became the President.
(Side note: One may recall, if one is a big useless nerd like me, that Leela also wound up on Gallifrey earlier in the fourth Doctor's tenure. Given their love of obscure continuity details, the New Adventures authors certainly wouldn't let that slide, so they decided that she became Romana's bodyguard. This in turn led the usual suspects to posit some sort of pairing between Romana and Leela, an idea which is
Anyway, all that aside there was some political wrangling and a lot of arguing and yelling and then the Time War happened and Gallifrey was destroyed, so Romana's probably dead.
Which makes me sad, so I prefer to believe that the novels are not canon, Romana was safely in E-Space during the war, and someday will come back. Yes! Someday! There is more terrible fanfiction yet to be written!
(As for Lalla Ward herself, she played Ophelia in "Hamlet" and wrote some books and later married Richard Dawkins. So that's also pretty cool, to be honest.)