Damn you, I was going to say she was Ace you nerdy ninja! God I hated Ace.LorChan said:Why would Jenny know his name?
I just think River Song is her own character. If I wanted to get into fan-wank territory I'd say Romana, because I adore Romana, but she isn't like any Time Lord character we've yet met in who-dom. Maybe she's a Time Lord, but not a former companion.
Closest thing I can think of is Ace, who (depending on which spin offs you read) may or may not have gone to Gallifrey to become a Time Lord. River's personality is a little like Ace, and if Ace became a Time Lord (how does that work, anyway?) before the Time War began, it would fit with continuity. Ace certainly wouldn't obey the non-interference laws after any time with the Doctor.
Of course, this whole theory depends on the expanded universe being canon.
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OK I'm going to put forward the suggestion that River Song is Leela even though she is nothing like Leela, except that they are both killers. Leela left the Doctor to stay on Gallifrey with Andred so that puts her in the mix to learn Tardis piloting etc. The problem with any older companions is that River Song said the tenth Doctor looked young, so I think she may have her own story. I'll hold off judgement until River Song kills someone with a Janis Thorn though.