3. His Relationship With River Song
Let me start by saying that this post is not supposed to imply that River will never come back to the show. Steven Moffat has said that he would bring River back if it was for a good story, so there is a possibility that we'll see her with the Twelfth Doctor. Personally, I don't think that's a good idea. Her send-off in The Name of the Doctor was a perfect ending to the relationship between her and Eleven. That relationship was one of the defining factors of Eleven's era. Before Eleven, she was just this unknown person that knew the Doctor at a future point in his timeline. With Eleven, she became a fleshed-out character who who added excitement, spice and a bit of sultriness to Eleven's adventures, all the while keeping some of that mystery of who she was until season 6. Some will say that River's storyline in season 6 was either detrimental to the season, or the reveal of her background was a let-down. With the wibbley wobbley timey-wimey nature of Doctor Who, I won't deny that there was room for improvement when it came to her timeline, but I thought the reveal of her being Amy and Rory's daughter was a great twist. Still, it's important to focus on her as a person. Here we have a woman who was raised to kill the Doctor from a young age, and she very nearly succeeded had it not been for her brief glimpse at the woman she was to become in Let's Kill Hitler. We have a woman who was able to overcome her programming and start on the path to becoming a better person, albeit a slightly unhinged psychopath. It was also interesting from a narrative perspective to have a character who was one of the few people who knew more than the Doctor did because she had experienced events before he had. Because of that, the line "spoilers" will forever be primarily associated with Doctor Who for me. The most important thing that I believe is that the Doctor truly cared about River. Naturally he cares about every one of his companions, but I think that he and River and such great chemistry together that it sets them apart from the other companions. This was a woman who had been around the block herself and could handle herself in a tough situation. The two of them together were like rambunctious kids going off to explore a new area of the neighborhood; in this case a neighborhood being the entire universe. From viewing her with suspicion to viewing her with respect and admiration, the Doctor treasured his relationship with River, and it was mutually beneficial. She helped him in times of need, and he helped a younger version of her become a better person. By the time of their encounter in The Name of the Doctor, I believe that at that point he truly did love her. So, if we see her running off with the Twelfth Doctor in season 8, so be it, but nothing will ever replace the relationship that her and Eleven shared.