Romana fans (me, for instance) would beg to differ.
In fact, the Doctor’s grand-daughter - presumably his biological granddaughter and therefore presumably also a Time Lady herself - appeared onscreen before the Doctor himself in the very first episode.
There were plenty of minor Time Lady characters apart from those two major characters, as well.
Susan was definitely a Time Lady - the first description of Gallifrey (though referred to as only her homeworld) came from her in the serial The Sensorites - it’s similar to the one Tennant gave in The Sound of Drums.
The other major Time Lady is the recurring villain, the Rani.
Well, Romana was definitely a Time Lord (or Time Lady if you will), but the jury is still out on Susan. While (AFAWK) all Time Lords were from Gallifrey, not all Gallifreyans were Time Lords - they were an elite class who had to go through intensive training at an Academy. Susan was very young and it was always implied that she’d been travelling for as far as she could remember. She likely never went to the Academy, or had Time Lord abilities such as regeneration.
Other Time Ladies - Flavia, a Time Lady seen during “the Five Doctors” special (when, at the end of the special, the Doctor is appointed president of the Time Lord council, he foists the job off onto her), and the mysterious Inquisitor from the “Trial of a Time Lord” mega-serial.
Also, it was also implied in “the Doctor’s Wife” that Time Lords/Ladies can indeed change sex during regeneration. The Doctor describes his old friend “the Corsair” would have the same tattoo re-applied, every time he or she regenerated, no matter what sex s/he was.
Anyway, that’s enough geeking out for me. Congrats to Mr. & Mrs. Tenant (or Mr. Tenant & Ms. Moffat, whatever the case may be.)
Let this video be your moment of zen to round out the year. Watch as David Tennant narrates a group of flying Rays as they leap out of the Sea of Cortez and experience a quick moment of flight.
(I’m of the opinion that they are indeed driving their prey as suggested. It looks like they go out of their way to make the biggest splash possible rather than reentering the water more gracefully.)