Romana II: Romana Harder (played by Lalla Ward) was the sexy incarnation of Romana and definitely the best . She regenerated from Romana 1 after Romana 1 realized she wasn't the sexiest Romana. She later regenerated into Romana 3, twice, except then she didn't.
She travelled with the Fourth Doctor, K-9 and even put up with Adric for awhile. Tom Baker and Lalla were banging IRL whilst working on Doctor Who and actually got married in 1980 but the marriage only lasted 16 months. Lalla was later married to famed-fedora tipper Richard Dawkins but not anymore :(
First appeared in Destiny of the Daleks though Lalla had already been in The Armageddon Factor where Tom had taken a shine to her and gone on to demand she become the new companion. Based Tom, the man with the plan.
Romana was the cutest companion around and famously wore a schoolgirl outfit while on a kinky trip with Tom Baker to Paris during the story City of Death. Technically, she had no reason to hang around after The Key to Time was found, but would you kick her out if she decided to loiter around?
Romana decided to stay in E-space in the story Warriors' Gate. Tom was depressed... for about 10 minutes (which is the longest he's ever been depressed for in his life) until he realised Adric was still his companion, at which point he became positively suicidal. He was dead two stories later. Bravo Adric.
After Who Edit
As for her personal life, she went from one drunken English madman to another.