“I live for the days when I see him. The day is coming when I’ll look into that man’s eyes—my Doctor—and he won’t have the faintest idea who I am. And I think it’s going to kill me.”
When River Song arrived, she was like no one else. She called the Doctor and he came at her word. She knew his name, a secret beyond a secret. And most incredibly of all, she knew more than he did.
The relationship between the Doctor and his companions is one of balance, and therefore one of inherent inequality. That’s not necessarily bad—two people can be on different levels without having less value than the other. The Doctor is brilliant, a genius of unrivaled proportions who always knows more than anyone else. The companions don’t know shit about technology or science, but they can feel the awe and compassion that the Doctor’s age and knowledge has isolated him from.
River shattered this pattern to bits. She knew more about the Doctor than anyone, was a doctor herself. She literally beat the Doctor into submission. She came tumbling out of the sky, all on her own, with every confidence that the Doctor would catch her, because she’d asked him to.
River was amazing, the first truly badass companion (Jack Harkness aside). But the problems that have played out in her character and arc were there from the beginning. River’s position as the Doctor’s equal, someone who rivaled him both in knowledge and emotion, was untenable from the start.
The essential problem is that River is not a full-time companion. There are a whole slew of things about River that would come to light if she were living on the Tardis, and none of them would allow her to be a companion. River’s violence suits her well when she’s on her own, or at the Doctor’s behest, but the Doctor is a man who abhors guns, hates violence. His openness to River’s violence is a sign that something is seriously wrong, which plays out to its logical and horrible conclusion at Demon’s Run.
And River’s knowledge, which comes from the fact that she lives in a different point in her timeline then the Doctor, was a scale that was bound to unbalance. When River appeared at her start, not knowing anything but indiscriminate violence, she was fun but stupid. She nearly got herself killed several times, she wreaked havoc through history, she practically ran over her parents. She then nearly destroyed all of time and space in a fruitless gesture to show her love, endangering all of reality.
Finally, it becomes clear that River is not living apart from the Doctor voluntarily, in order to have her own life. As becomes clear in the quote at the beginning, River’s love for the Doctor means her entire life revolves around stalking him through time and space. She isn’t leading a life somewhere and joining up with the Doctor for adventures: she’s passing time until she sees him again.
River’s relationship with the Doctor is revealed to be more unequal than that of any previous companion, because her entire life revolves around the Doctor, while his life is only tangentally related to hers. River chose her profession because of the Doctor, she drops everything whenever he turns up, while he can wait until its convenient to go see her. He was even instrumental in her birth!
As if that were not enough, the Doctor constantly demands sacrifices of River far greater than any demanded of any other Companion, except maybe Donna. River was always destined to die in his place, she went to prison so the Doctor could screw with his reputation, she even lost her parents because of the Doctor. It is because of his lack of ability to find a freaking time lord on Earth that she was victimized by the Silence, turned into a sub-human.
River started out as the coolest, most powerful lady every to saunter across the screen of Doctor Who. As it stands now, she is the most damaged and dependent of any companion the Doctor ever had. Dammit.