Let's look at the facts, shall we?
- Her sister is The Rani.
- She is much older than she looks. The first time her sister shows up, Patsy admits that she has absolutely no idea what her own age is. But she's at least in her seventies, as she is the older sister, and that's how old Rani is. She certainly doesn't look it, and Time Lords age much slower than humans.
- She can survive things that no human should. Like The Doctor, she has been exposed to lethal amounts of drugs (it took an ungodly amount of anesthesia to knock the Doctor out in the tv movie) and poison (he can counteract it) and lived. Patsy consumes fatal amounts of random chemicals on a daily basis, and yet seems to be unkillable. She also slept soundly in a burning room for a whole night without suffocating. Time Lords have a "respiratory bypass system" that lets them survive without oxygen for some time.
- She protects Time and Space. In Sapphire and Steel, Patsy (under a pseudonym) is a mysterious Time Agent (or something) who has telepathic powers (just like the Doctor). And that show was pretty much explicitly ATV's answer to Doctor Who.
- In The Curse of Fatal Death, the Doctor regenerates several times in a row. His last form is...Patsy! And Saffron is his/her companion!