He was always rather partial to his knight. He would move his pieces around the board, sacrificing other, more powerful players if his knight was in danger. When he played with Harry or Hermione, this strange loyalty would go unnoticed, as they were always concentrating much to hard on their own side to pay attention to his (which was perhaps why they always lost). Only when he was playing at home in the Burrow with his father, who had taught him the game, would comments be made. 'Ron,' he would say, while scraping the remains of Ron's queen off the board, 'you could have let me take your knight and have had your queen free to come and check my king.' Ron's reply was always the same. A shrug and 'I can win without her.'
And it was true. Many years had past since Ron won his first game against Mr. Weasley. It had been a brilliant match, a full two hours, that had concluded with the entire Weasley family gathered in the living room huddled around Ron's side of the board (somewhat to Mr. Weasley's dismay) cheering on his two pawns and lone rook. Since then, winning with just a few pieces had been Ron's trademark style. It made the game longer, harder, more exciting. For a while, the rook, commemorative of his first victory, had been Ron's favorite piece. But that first year at Hogwarts and the greatest chess game of his life had changed that; it had changed him.
He had never been very introspective. He chose to focus on the reality of the present, what was actually going on around him. What homework he had to do, what was for dinner, how about some Quidditch? But he came to notice that when he simply skimmed the surface, he missed a lot of details and often ended up looking the fool (usually in front of Hermione). That was the rook's job after all; to sweep broadly around the board, skimming the edges, looking for breakouts and missing much of the subtler aspects of the game. Knights are different. They move in small, strategic patterns, slowly advancing from one side of the board to the other, never too fast, taking in all their surroundings, slowly analyzing their position. He never felt he had the subtlety.
But he found he had theloyalty. After the Great Chess Game where he first envisioned himself as a knight, and the race for the Philosopher's Stone, it became apparent that Harry was somewhere above them all. Not in a god-like sense, just in a sense that there were certain things he would have to do, things that his past had primed him for, things that were not entrusted, for whatever reason, to anyone else. This didn't bother Ron. He would never have asked for Harry's responsibility. He was quite content to assume the role of the knight who added the master, an unfaltering companion and a steadying hand.
It wasn't that he stopped being a rook. It was just that he had grown up, found that he could be something he thought he couldn't. He had come into a life of purpose, dignity and strength, each year at Hogwarts making him re-evaluate who and what he was and who and what he loved. Knights are supposed to protect their master and their lady. And, he thought as he gritted his teeth and sat wedged between Harry and Hermione during the silent ride to Godric's Hollow, that's exactly what he was going to do.